Summary:1. add information about automake and python-dev apt dependencies
2. Suggest current watchman version (4.5 vs 4.1)
Closes https://github.com/facebook/react-native/pull/6784
Differential Revision: D3161652
fb-gh-sync-id: 2a13b315fcb6f1898a5b3be12ddf15ed457befdb
fbshipit-source-id: 2a13b315fcb6f1898a5b3be12ddf15ed457befdb