Readme and doc clarification about local git repos

First of all, thank you for the great plugin!

This pull request contains short update to doc and `README.md` on how to use local git repo as a plugin.
Recently I was working on a plugin of my own. I didn't know how to test local updates I made. The work around is to push updates to remote repo (master branch), and then load that repo with Bundle. That felt like too much work.

Making updates to `~/.vim/bundles/plugin/` is too messy.

The solution to this is pretty simple - you just specify a path to local git repo like this (note the path is absolute):
`Bundle 'file:///Users/gmarik/path/to/plugin'`

It took me more than one hour of exploring the vundle documentation, source code and googling to "find the solution". I figured an update to documentation would save that time to other people that want to do the same thing.

Hope this is helpful!