Update CONTRIBUTING.md
Update CONTRIBUTING.md

Summary: Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: strulovich

Differential Revision: D34512702

fbshipit-source-id: f400ba11d0f83e254e93eb96fb56d4ff9d304b74

facebookincubator/ktfmt,facebookincubator/ktfmt,facebookincubator/ktfmt,facebookincubator/ktfmt,facebookincubator/ktfmt