Update documentation for git based installation

Eclipse Thym bug [1] enhances the cordova plugin discovery dialog so that the dialog can now accept urls that specify git-refs (tags, branches) and a sub directory as described on Cordova documentation [2]. We need to update our docs so that the format for such git urls are explained.

[1] 441910: Support for git-ref and sub directory is incomplete for installing Cordova plug-ins
https://bugs.eclipse.org/bugs/show_bug.cgi?id=441910

[2] http://cordova.apache.org/docs/en/3.5.0/guide_cli_index.md.html#The%20Command-Line%20Interface