Advanced readme

I have done a rewrite of the README and converted it to MarkDown allowing more edition options.

I think this version is more concise and ruby knowers oriented, feel free to reject, modify or pick only some parts.

You may like to pick the part about using Sinatra, Sinatra-AssetPack and Sinatra-Static, simple instruction one how to manage assets in a more powerfull way (ex. versionning assets ala styles.73821.css & logo.21321.png) in combination with this gem.