[README] Add link to react-native tag on Stack Overflow

Raise some awareness of the tag and let people know they can help by answering qquestions and be altruistic and helpful.

README.md

@@ -80,8 +80,9 @@ The GitHub issues are intended for bug reports and feature requests. For help an
 
 ## Contributing
 
-For more information about contributing, see our [Contribution Guidelines](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md).
+For more information about contributing PRs and issues, see our [Contribution Guidelines](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md).
 
+We encourage the community to ask and answer questions on Stack Overflow with [the react-native tag](https://stackoverflow.com/questions/tagged/react-native). It's a great way to help out and be involved!
 
 ## License