Add gitter.im chat badge
Add gitter.im chat badge

Summary:
Add a badge to the readme.

Fix #179

Reviewed By: IanChilds

Differential Revision: D5044348

fbshipit-source-id: 6ae973137755a8b83b66ab80a0af2500e1cf43ab

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