chore: Update PULL_REQUEST_TEMPLATE.md

Add requirements around documentation

Author: marcoscaceres

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,4 +8,9 @@ Implementation commitment:
 
  * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
  * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
- * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
+ * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
+
+Documentation (new feature):
+
+ * [] Pinged MDN
+ * [] Added example to README or spec