Summary:
Copy over the existing CoC from https://code.facebook.com/codeofconduct into the repo.

N/A

[GENERAL][FEATURE][./CODE_OF_CONDUCT.md] - Copy FB OSS CoC into repo.
Closes https://github.com/facebook/react-native/pull/16465

Differential Revision: D6167348

Pulled By: hramos

fbshipit-source-id: 5ca70a46bf7bded872c20ceb5a5941cc8d2be495