[guide] Add a 19.9 bullet in a Whitespace section

- with explanations about preventing two blank lines from  appearing consecutively in JavaScript code

Author: Ensive

README.md

@@ -2656,8 +2656,63 @@ Other Style Guides
     }
     ```
 
+  <a name="whitespace--no-multiple-blanks"></a>
+  - [19.9](#whitespace--no-multiple-blanks) Do not use multiple blank lines to pad your code. eslint: [`no-multiple-empty-lines`](https://eslint.org/docs/rules/no-multiple-empty-lines)
+
+    <!-- markdownlint-disable MD012 -->
+    ```javascript
+    // bad
+    class Person {
+      constructor(fullName, email, birthday) {
+        this.fullName = fullName;
+
+
+        this.email = email;
+
+
+        this.setAge(birthday);
+      }
+
+
+      setAge(birthday) {
+        const today = new Date();
+
+
+        const age = this.getAge(today, birthday);
+
+
+        this.age = age;
+      }
+
+
+      getAge(today, birthday) {
+        // ..
+      }
+    }
+
+    // good
+    class Person {
+      constructor(fullName, email, birthday) {
+        this.fullName = fullName;
+        this.email = email;
+        this.setAge(birthday);
+      }
+
+
+      setAge(birthday) {
+        const today = new Date();
+        const age = getAge(today, birthday);
+        this.age = age;
+      }
+
+      getAge(today, birthday) {
+        // ..
+      }
+    }
+    ```
+
   <a name="whitespace--in-parens"></a><a name="18.9"></a>
-  - [19.9](#whitespace--in-parens) Do not add spaces inside parentheses. eslint: [`space-in-parens`](https://eslint.org/docs/rules/space-in-parens.html)
+  - [19.10](#whitespace--in-parens) Do not add spaces inside parentheses. eslint: [`space-in-parens`](https://eslint.org/docs/rules/space-in-parens.html)
 
     ```javascript
     // bad
@@ -2682,7 +2737,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--in-brackets"></a><a name="18.10"></a>
-  - [19.10](#whitespace--in-brackets) Do not add spaces inside brackets. eslint: [`array-bracket-spacing`](https://eslint.org/docs/rules/array-bracket-spacing.html)
+  - [19.11](#whitespace--in-brackets) Do not add spaces inside brackets. eslint: [`array-bracket-spacing`](https://eslint.org/docs/rules/array-bracket-spacing.html)
 
     ```javascript
     // bad
@@ -2695,7 +2750,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--in-braces"></a><a name="18.11"></a>
-  - [19.11](#whitespace--in-braces) Add spaces inside curly braces. eslint: [`object-curly-spacing`](https://eslint.org/docs/rules/object-curly-spacing.html)
+  - [19.12](#whitespace--in-braces) Add spaces inside curly braces. eslint: [`object-curly-spacing`](https://eslint.org/docs/rules/object-curly-spacing.html)
 
     ```javascript
     // bad
@@ -2706,7 +2761,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--max-len"></a><a name="18.12"></a>
-  - [19.12](#whitespace--max-len) Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per [above](#strings--line-length), long strings are exempt from this rule, and should not be broken up. eslint: [`max-len`](https://eslint.org/docs/rules/max-len.html)
+  - [19.13](#whitespace--max-len) Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per [above](#strings--line-length), long strings are exempt from this rule, and should not be broken up. eslint: [`max-len`](https://eslint.org/docs/rules/max-len.html)
 
     > Why? This ensures readability and maintainability.
 
@@ -2736,7 +2791,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--block-spacing"></a>
-  - [19.13](#whitespace--block-spacing) Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: [`block-spacing`](https://eslint.org/docs/rules/block-spacing)
+  - [19.14](#whitespace--block-spacing) Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: [`block-spacing`](https://eslint.org/docs/rules/block-spacing)
 
     ```javascript
     // bad
@@ -2749,7 +2804,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--comma-spacing"></a>
-  - [19.14](#whitespace--comma-spacing) Avoid spaces before commas and require a space after commas. eslint: [`comma-spacing`](https://eslint.org/docs/rules/comma-spacing)
+  - [19.15](#whitespace--comma-spacing) Avoid spaces before commas and require a space after commas. eslint: [`comma-spacing`](https://eslint.org/docs/rules/comma-spacing)
 
     ```javascript
     // bad
@@ -2762,7 +2817,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--computed-property-spacing"></a>
-  - [19.15](#whitespace--computed-property-spacing) Enforce spacing inside of computed property brackets. eslint: [`computed-property-spacing`](https://eslint.org/docs/rules/computed-property-spacing)
+  - [19.16](#whitespace--computed-property-spacing) Enforce spacing inside of computed property brackets. eslint: [`computed-property-spacing`](https://eslint.org/docs/rules/computed-property-spacing)
 
     ```javascript
     // bad
@@ -2779,7 +2834,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--func-call-spacing"></a>
-  - [19.16](#whitespace--func-call-spacing) Avoid spaces between functions and their invocations. eslint: [`func-call-spacing`](https://eslint.org/docs/rules/func-call-spacing)
+  - [19.17](#whitespace--func-call-spacing) Avoid spaces between functions and their invocations. eslint: [`func-call-spacing`](https://eslint.org/docs/rules/func-call-spacing)
 
     ```javascript
     // bad
@@ -2793,7 +2848,7 @@ Other Style Guides
     ```
 
   <a name="whitespace--key-spacing"></a>
-  - [19.17](#whitespace--key-spacing) Enforce spacing between keys and values in object literal properties. eslint: [`key-spacing`](https://eslint.org/docs/rules/key-spacing)
+  - [19.18](#whitespace--key-spacing) Enforce spacing between keys and values in object literal properties. eslint: [`key-spacing`](https://eslint.org/docs/rules/key-spacing)
 
     ```javascript
     // bad
@@ -2805,10 +2860,10 @@ Other Style Guides
     ```
 
   <a name="whitespace--no-trailing-spaces"></a>
-  - [19.18](#whitespace--no-trailing-spaces) Avoid trailing spaces at the end of lines. eslint: [`no-trailing-spaces`](https://eslint.org/docs/rules/no-trailing-spaces)
+  - [19.19](#whitespace--no-trailing-spaces) Avoid trailing spaces at the end of lines. eslint: [`no-trailing-spaces`](https://eslint.org/docs/rules/no-trailing-spaces)
 
   <a name="whitespace--no-multiple-empty-lines"></a>
-  - [19.19](#whitespace--no-multiple-empty-lines) Avoid multiple empty lines and only allow one newline at the end of files. eslint: [`no-multiple-empty-lines`](https://eslint.org/docs/rules/no-multiple-empty-lines)
+  - [19.20](#whitespace--no-multiple-empty-lines) Avoid multiple empty lines and only allow one newline at the end of files. eslint: [`no-multiple-empty-lines`](https://eslint.org/docs/rules/no-multiple-empty-lines)
 
     <!-- markdownlint-disable MD012 -->
     ```javascript