Summary:
I've seen quite a few PRs try to change sync'ed files so let's add this warning and hope people read it.
Not sure if it's better / possible to put it in the oss folder instead.
Pull Request resolved: https://github.com/facebook/react-native/pull/21251
Differential Revision: D9993401
Pulled By: hramos
fbshipit-source-id: 05d22c7f8da1eba97a83d2b7dbc66d9324695a1b