mention RNTester app in contributor guide (facebook#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: facebook#28042

Test Plan: not necessary

Reviewed By: TheSavior

Differential Revision: D20401260

Pulled By: hramos

fbshipit-source-id: 01c1b7dff56b59909c94b2feb609650f0baba1a9

CONTRIBUTING.md

@@ -72,18 +72,21 @@ We recommend referring to the [CONTRIBUTING](https://github.com/facebook/react-n
 
 ## Contributing Code
 
-Code-level contributions to React Native generally come in the form of [pull requests](https://help.github.com/en/articles/about-pull-requests). The process of proposing a change to React Native can be summarized as follows:
+Code-level contributions to React Native generally come in the form of [pull requests](https://help.github.com/en/articles/about-pull-requests). These are done by forking the repo and making changes locally. Directly in the repo, there is the [`RNTester` app](/RNTester) that you can install on your device (or simulators) and use to test the changes you're making to React Native sources.
+
+The process of proposing a change to React Native can be summarized as follows:
 
 1. Fork the React Native repository and create your branch from `master`.
-2. If you've added code that should be tested, add tests.
-3. If you've changed APIs, update the documentation.
-4. Ensure the test suite passes, either locally or on CI once you opened a pull request.
-5. Make sure your code lints (for example via `yarn lint --fix`).
-6. Push the changes to your fork.
-7. Create a pull request to the React Native repository.
-8. Review and address comments on your pull request.
+2. Make the desired changes to React Native sources. Use the `RNTester` app to test them out.
+3. If you've added code that should be tested, add tests.
+4. If you've changed APIs, update the documentation, which lives in [another repo](https://github.com/facebook/react-native-website/).
+5. Ensure the test suite passes, either locally or on CI once you opened a pull request.
+6. Make sure your code lints (for example via `yarn lint --fix`).
+7. Push the changes to your fork.
+8. Create a pull request to the React Native repository.
+9. Review and address comments on your pull request.
     1. A bot may comment with suggestions. Generally we ask you to resolve these first before a maintainer will review your code.
-9. If you haven't already, please complete the [Contributor License Agreement](https://github.com/facebook/react-native/wiki/Contributor-License-Agreement) ("CLA"). **[Complete your CLA here.](https://code.facebook.com/cla)**
+10. If you haven't already, please complete the [Contributor License Agreement](https://github.com/facebook/react-native/wiki/Contributor-License-Agreement) ("CLA"). **[Complete your CLA here.](https://code.facebook.com/cla)**
 
 If all goes well, your pull request will be merged. If it is not merged, maintainers will do their best to explain the reason why.
 

RNTester/README.md

@@ -8,7 +8,7 @@ Before running the app, make sure you ran:
 
     git clone https://github.com/facebook/react-native.git
     cd react-native
-    npm install
+    yarn install
 
 ### Running on iOS