update contributing documentation

https://leap.se/en/docs/get-involved/coding should point to https://bitmask.readthedocs.io/en/latest/hacking/index.html or https://bitmask.readthedocs.io/en/latest/hacking/index.html.

Maybe the README should link the contributing doc more clearly.

What is the recommended way to do MRs? Can a new commer create them in 0xacab? Or do we need to tell them to do it in github?