Documentation cleanup

The documentation is scattered across the README and http://di-in-scala.github.io/.
I suggest that everything that is on the documentation website be removed from the README.

I guess that one reason why this hasn't been done is because the website is not only about macwire but about dependency injection in scala. Unfortunately this is quite a broad topic and it turns out that the guide is mostly about macwire with a compilation of links to other frameworks, github projects or blog posts.

So I would do either one of these:

1. change the name of the documentation website into macwire documentation and move everything that's not directly about macwire (that is, everything up to "Running example"), into stack overflow.
2. same as 1 but leave the link compilation in http://di-in-scala.github.io/

My preference goes to 1) as the second option would leave us with a rather skinny website.

Compare the README with the documentation at http://di-in-scala.github.io/, paying attention to the material before “Running example” and the links to other frameworks and projects. Review the issue discussion before choosing between the two proposed reorganizations. Done means the README and documentation site have a clear, non-duplicated scope and the selected content has been moved or removed consistently.