mention RNTester app in contributor guide (#28042)

Summary:
motivation is following - the RNTester app is imho the best place to try out any changes that a contributor would make, yet it is not directly mentioned in the contributor guide. This fixes it.

## Changelog

[Internal] - Docs
Pull Request resolved: https://github.com/facebook/react-native/pull/28042

Test Plan: not necessary

Reviewed By: TheSavior

Differential Revision: D20401260

Pulled By: hramos

fbshipit-source-id: 01c1b7dff56b59909c94b2feb609650f0baba1a9