Summary:
Make it more obvious that pull requests should generally target the `master` branch. Provide visible clarity in both the `CONTRIBUTING.md` and the actual pull request template.
Closes https://github.com/facebook/react-native/pull/8748

Differential Revision: D3566827

fbshipit-source-id: 6326f18b93594e928b1c4e0d09a739a8ccedaa62