Readme tweaks
Readme tweaks

Summary: added more info and docs links to the readme

Reviewed By: passy

Differential Revision: D4876453

fbshipit-source-id: 150db4b96ab8f34e7c14d065346923b0ed2098c9

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho