Update README.md text regarding broken symlinks in FacebookSDK.framework

I agree with @georgir https://github.com/Wizcorp/phonegap-facebook-plugin/issues/545#issuecomment-59198438 that the current README.md isn't very accurate regarding the installation and broken symlinks.


Attempted to add some better explanatory text, atleast until the plugin is perhaps published as a tar-ball as suggested in [CB-6092](https://issues.apache.org/jira/browse/CB-6092).