Improve CONTRIBUTING.md
Improve CONTRIBUTING.md

Summary:
Made it easier to reach the .clang-format file by adding a link to it.
Closes https://github.com/facebook/redex/pull/267

Reviewed By: maxmg22

Differential Revision: D6010865

Pulled By: justinjhendrick

fbshipit-source-id: 71180d0727b2886198e57c582ce9c7f7a8e336a6

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