improve CONTRIBUTING.md about Pull Request

Summary:
**motivation**

Lots of people does not sure what to do after a PR.
common questions like:

* should I keep rebasing this PR to the latest master branch ?
* should I keep squashing all commits in a PR into single one?

This may clear it a bit and makes people less nervous :scream:

**Test plan (required)**
Closes https://github.com/facebook/react-native/pull/8081

Differential Revision: D3424076

fbshipit-source-id: 6c5f1b0b2f0bee03c647c2f8cad00b788130a5fc