Add warning in README

Author: bhch

README.md

@@ -12,4 +12,13 @@ https://bhch.github.io/react-json-form/playground/
 
 https://bhch.github.io/react-json-form/
 
+###  ⚠️ Important notes
+
+ 1. **Consider this library as a work-in-progress** (at least until version 3 which will be a more stable release).
+ 2. Currently, this library doesn't provide default CSS styles. So, you're required to write
+ your own CSS styles. You can also copy the styles from the demo page (see [`docs.css` after `Line 433`](https://github.com/bhch/react-json-form/blob/master/docs/static/css/docs.css#L433)) as a starting point.
+ 3. Be prepared for breaking changes regarding UI structure and CSS class names. Currently, CSS class names don't
+ follow a particular naming standard. But this will change in v3.
+ 4. Support for UI themes will be added soon.
+
 [npm]: https://www.npmjs.com/package/@bhch/react-json-form/