Add contributing guide

[OriolAbril]

I think for arviz-base we can start with a single page on the docs that is a simplified version of combining https://arviz-plots.readthedocs.io/en/latest/contributing/testing.html and https://arviz-plots.readthedocs.io/en/latest/contributing/docs.html.

It should also warn about the local docstub check. That is, pre-commit needs to run on an env with docstub installed. If it is installed so it runs automatically you'll only be able to commit when an env with docstub is active, otherwise you should be able to always run tox -e check (which creates its own env on which to run pre-commit)