Update Github Readme
Update Github Readme

Summary: We had lots of issues submitted with "X not found"

Reviewed By: int3

Differential Revision: D5327548

fbshipit-source-id: d06056b8f187f19f699ed2d6e7fa676766de7fc1

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex