Developer guidelines

A short document with development guidelines. First draft, but ready for some feedback.
Really? It's just a PEP8 rule, so either way it's a guideline that is implied by saying we follow PEP8.

If you are referring to mathematical notation, I can add a sentence along these lines: "Variable names that are common mathematical notation are acceptable e.g. `p` for probability, `x` and `y` for in- and output, `W` for weights, etc.