Update contribution and style guidelines

Following the initial discussion in #41 (closed) this is an attempt at updating the contribution guide for ramsis. It's a first draft which should be seen as a basis for discussion. OTOH, I think it's reasonable trying not to spend too much time on this.

This ended up becoming only a small update to the original guide, since I think the original already covered most things nicely. I added a few clarifications here and there and tried to include rationale where necessary. I would prefer to stay broad in the guideline and not to outline every stylistic detail. After all, PEP8 covers most of that anyway.

My main premise is outlined in the first paragraph: Try to stick to established standards where possible, don't reinvent the wheel.