Docs: Add new experimental syntax policy to README (fixes #9804) (#10408

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

See #9804 (comment).

* Docs: Small tweak for friendliness

* Docs: Clarified that no-crash PRs will only be accepted for experimental features represented in ESTree

README.md

@@ -145,7 +145,9 @@ 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 (as long as they are implemented using the correct experimental ESTree syntax). 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), we recommend you 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.