Summary:**Motivation:** I don't like storing credentials in config files and a safer solution is to store them in the Keychain Access app.
Therefore I added a link that describes how one may go about to store the passwords in the Keychain Access app in OSX instead.
Found a [stackoverflow post](http://stackoverflow.com/a/24480579/1836121) but it wasn't very detailed so I decided to write it up myself. I understand if you have some policy against external links to blogs and I guess we could link to the stackoverflow explanation then instead.
Closes https://github.com/facebook/react-native/pull/7089
Differential Revision: D3207215
fb-gh-sync-id: 8011b55efe41ec6d54ad665e0e6a79cb9d806779
fbshipit-source-id: 8011b55efe41ec6d54ad665e0e6a79cb9d806779