Update Podfile documentation for RN >= 0.42.0

Summary:
I've updated the documentation that gives an example of Podfile adding a note that from React Native 0.42.0, users have to explicitly include Yoga in their Podfile.

I didn't want to modify existing `Podfile` from this documentation because they refer to `Podfile` from existing projects that do not use RN 0.42.0. So I just added a note and an example to explain how to include Yoga.

This PR is to fix the following issue: https://github.com/facebook/react-native/issues/12670
Closes https://github.com/facebook/react-native/pull/12728

Differential Revision: D4716172

Pulled By: hramos

fbshipit-source-id: 12a6e61b0d426c776a7cc2dea3ac22a50ffe56ef