Update CONTRIBUTING.md (#31590)

Summary:
Add more detail about requesting review in the contribution guide.
In the GAAD initiative it's particularly useful to know when certain PRs need our attention so wanted to highlight the code review process to remind contributors to request reviewers.

## Changelog

[General] [Added] - Added more detail about the review process

Pull Request resolved: https://github.com/facebook/react-native/pull/31590

Test Plan: Grammar, clarity review

Reviewed By: TheSavior

Differential Revision: D28719869

Pulled By: lunaleaps

fbshipit-source-id: 9213fefbe602588f9e5cee7ba240b93bc7f291b5