[25] Add an "Optional Tests" section to the README

Author: ssilverman

README.md

@@ -65,6 +65,39 @@ widely used, while draft-03 is long since deprecated.
 If you see anything missing from the current supported drafts, or incorrect
 on any draft still accepting bug fixes, please file an issue or submit a PR.
 
+Optional Tests
+--------------
+
+Tests in the `optional/` directory test optional validation behavior. Validators
+only need to execute tests for implemented features.
+
+There are different sets of optional features for each draft. They are
+summarized below.
+
+### Draft 2019-09
+
+1. Arbitrary-precision number support
+2. Encoded content in a string
+3. ECMA 262 regex quirks
+4. References to unknown keywords
+5. Numbers having zero-valued fractional parts are integers
+6. All the defined "format" values
+
+### Draft 07
+
+1. Arbitrary-precision number support
+2. Encoded content in a string
+3. ECMA 262 regex quirks
+4. Numbers having zero-valued fractional parts are integers
+5. All the "format" values
+
+### Drafts 06, 04, and 03
+
+1. Arbitrary-precision number support
+2. ECMA 262 regex quirks
+3. Numbers having zero-valued fractional parts are integers
+4. All the "format" values
+
 Who Uses the Test Suite
 -----------------------