Updated README install/usage instructions.

Hi,

First off, thanks for your work on this compass plugin!

Being new to the compass plugin world and this being first installation of one, I did not realise you had to not only import the plugin in your scss/sass files but also 'require' it in the compass ruby configuration file so that compass can add it to the sass load paths. Failing to do this receives the following error: 

> File to import not found or unreadable: h5bp.

I've simply updated the README to make this clearer.

* Added extra usage description to help those new to the compass plugin system.
* Made some small tweaks to installation instructions for readability. 
* Fixed some capitalisation.

Thanks again for your work!

Darian