Add section to README or ROADMAP about supported/unsupported features

This would help people figure out if they can use this library, and how best to contribute.

Could be useful to categorize into:
- Supported (working)
- Roadmap (working on it or will in near future): #518
- Unsupported (could possibly implement in the future, no current plans): #564 , #401 
- WONTFIX (will not implement): #492 (because no rendering)