Summary:
Moves `.clang-format` to repo root to allow for easier discoverability, and integration with auto-format plugins for editors and IDEs.
Pull Request resolved: https://github.com/facebook/react-native/pull/23328

Differential Revision: D13986715

Pulled By: davidaurelio

fbshipit-source-id: fcfda59842da10cd4bc02e4550b74782fbb59e0d