Add a Getting Started page

[mpwp]

The idea is to have an overview of the informations availble in order to use Yi.
Sort of sitemap which point to how to install, how to configure, how to how to...
Like a basic description of the global procedure to have an awesome working Yi in a second.

[ghost]

PRs always welcome :) I've been busy with other things, but I would be happy to review and merge.