Improve README file

The current README file lacks good installation instructions (#28), has an incomplete requirements section, and could do with a bit more detail about the available recipes and attributes.

It's also not in Markdown format, but that can come in a future pull request :wink:

I've rewritten the README file to have more information and follow the format in `opscode-cookbooks/nginx`, `opscode-mysql` (Opscode's [most-forked](https://github.com/search?o=desc&q=user%3Aopscode-cookbooks&s=forks&type=Repositories) and [most-starred](https://github.com/search?o=desc&q=user%3Aopscode-cookbooks&ref=cmdform&s=stars&type=Repositories) cookbooks), and `fnichol/chef-rvm`, which has great installation instructions.