[guide] [react] add comment about what JS standards are followed

Seen a few pull requests for the React style guide that try to introduce features that aren't yet in stage >= 3 (static class fields for i.e), so I've decided to add a comment to the beginning of the guide. Not sure if this is the right place to put it, but here it is.

This disclaimer might help:
1) give the community an idea what standards this guide is based on
2) prevent redundant pull requests

Author: raunofreiberg

react/README.md

@@ -2,6 +2,8 @@
 
 *A mostly reasonable approach to React and JSX*
 
+This style guide is mostly based on the standards that are currently prevalent in JavaScript, although some conventions (i.e async/await or static class fields) may still be included or prohibited on a case-by-case basis. Currently, anything prior to stage 3 is not included nor recommended in this guide.
+
 ## Table of Contents
 
   1. [Basic Rules](#basic-rules)