Add list of supported types for ContentBlock for docs
Add list of supported types for ContentBlock for docs

Summary:
This addresses the issue raised in #1051. I've just added a list of the types towards the top of documentation for ContentBlock.
Closes https://github.com/facebook/draft-js/pull/1459

Reviewed By: mitermayer

Differential Revision: D6163067

Pulled By: mitermayer

fbshipit-source-id: 9b8cc9e6da5079b35f2d4ab3caf447048090f052

facebook/draft-js