Re-structure documentation

[colszowka]

The README has become quite bloated. Though the introductory section seems to be quite simple to grasp, there's still a lot of support issues coming in from users, often pertaining to configuration trouble or missing the available config options.

It would probably be better for the README to explain the basics and then link to further guidelines sitting in docs/**/*.md and the API docs.

Having github/markup#84 fixed would be quite useful for this to avoid similar linking issues as with i.e. https://github.com/lsegal/yard or https://github.com/defunkt/resque, but for the time being the links probably should link straight to master on github.

[guiprav]

+1