Update PROGMEM.rst (esp8266#6872)

* Update PROGMEM.rst

Include array of strings in the description, since the examples in Arduino do not consider the right pointer size

* Update PROGMEM.rst

Author: LeisureLadi

doc/PROGMEM.rst

@@ -237,6 +237,42 @@ the value back.
       }
     }
 
+How do I declare Arrays of strings in PROGMEM and retrieve an element from it.
+------------------------------------------------------------------------------
+
+It is often convenient when working with large amounts of text, such as a project with an LCD display, to setup an array of strings. Because strings themselves are arrays, this is actually an example of a two-dimensional array.
+
+These tend to be large structures so putting them into program memory is often desirable. The code below illustrates the idea.
+
+.. code:: cpp
+
+    // Define Strings
+    const char string_0[] PROGMEM = "String 0";
+    const char string_1[] PROGMEM = "String 1";
+    const char string_2[] PROGMEM = "String 2";
+    const char string_3[] PROGMEM = "String 3";
+    const char string_4[] PROGMEM = "String 4";
+    const char string_5[] PROGMEM = "String 5";
+
+    // Initialize Table of Strings
+    const char* const string_table[] PROGMEM = { string_0, string_1, string_2, string_3, string_4, string_5 };
+
+    char buffer[30]; // buffer for reading the string to (needs to be large enough to take the longest string
+
+    void setup() {
+      Serial.begin(9600);
+      Serial.println("OK");
+    }
+
+    void loop() {
+      for (int i = 0; i < 6; i++) {
+        strcpy_P(buffer, (char*)pgm_read_dword(&(string_table[i])));
+        Serial.println(buffer);
+        delay(500);
+      }
+    }
+
+
 In summary
 ----------