Add contribution guidelines

[Globegitter]

Similar to how we have https://github.com/artificialio/sane/blob/master/CONTRIBUTING.md we need a conribution guideline so

1. new people know where and how they can easiest contribute to the book

2. all contributors know which format and style of writing to follow that the whole book is coherent.

[ErikCH]

You'll still except PRs before this is in place correct? We can always change the format in the future...

[Globegitter]

@ErikCH Yep a PR would be really appreciated in fact.