Add installation and usage text to README.md

I love this project as it solves multiple problems in my previous software deployment method.  Mine was git hook based, but did not benefit from the management layer provided by gitolite or a system like this to manage the hooks themselves.  

I updated the README.md to reflect the installation and usage text in the original post at http://demonastery.org/2012/09/a-hooking-system-for-gitolite/  I hope this draws together the latest code and enough documentation on how to install and use it.  

Philip