Move out of contributions instructions from README.md #19

A suggestion:
Maybe your frontpage should be a readme.md not a contributing.md?
I don't really need to know how to give feedback, help write a chapter or donate when I don't know what the book even is.
I guess that if you read through the first 4/5 pages of the introduction you get an idea of what the book is, but you should really have a one paragraph explanation of the project as the first thing people see.