Add installation instructions for MacOS X/Homebrew

I recently created a [formula for the Homebrew package manager](https://github.com/Homebrew/homebrew-completions/blob/master/maven-completion.rb) to allow easy installation of the maven-bash-completion on MacOS X. Would be nice if you could include the installation instructions in the README file, so that Mac users are aware of the easy installation method. Feel free to adapt the wording as you see fit...