Docs: Add new experimental syntax policy to README (fixes #9804)

See #9804 (comment).

README.md

@@ -146,7 +146,9 @@ ESLint has full support for ECMAScript 3, 5 (default), 2015, 2016, 2017, and 201
 
 ### What about experimental features?
 
-ESLint doesn't natively support experimental ECMAScript language features. You can use [babel-eslint](https://github.com/babel/babel-eslint) to use any option available in Babel.
+ESLint's parser only officially supports the latest final ECMAScript standard. We will make changes to core rules in order to avoid crashes on stage 3 ECMAScript syntax proposals. We may make changes to core rules to better work with language extensions (such as JSX, Flow, and TypeScript) on a case-by-case basis.
+
+In other cases (including if rules need to warn on more or fewer cases due to new syntax, rather than just not crashing), you should use other parsers and/or rule plugins. If you are using Babel, you can use the [babel-eslint](https://github.com/babel/babel-eslint) parser and [eslint-plugin-babel](https://github.com/babel/eslint-plugin-babel) to use any option available in Babel.
 
 Once a language feature has been adopted into the ECMAScript standard (stage 4 according to the [TC39 process](https://tc39.github.io/process-document/)), we will accept issues and pull requests related to the new feature, subject to our [contributing guidelines](https://eslint.org/docs/developer-guide/contributing). Until then, please use the appropriate parser and plugin(s) for your experimental feature.