Extract lengthy explanation into dedicated file

@charlierudolph @allewun 

This extracts the lengthy background information about Git Town into a dedicated file outside of the main Readme.
I want to keep the main Readme as a short, to the point, and efficient intro to Git Town. Too much bla bla only turns people off here, because nobody reads long texts anyways.

I have also done another iteration on a lot of the existing content, and added the preliminary Git Town logo (to be improved upon later, but its good enough for starting to get a feel for it online imho).

Please take a look and let me know what you think!
The reason is that bigger teams have a lot more issues than the few mentioned here, and Git Town solves many of them, and not only the few mentioned here.