Add documentation on how to build and run documentation locally

[MrPowers]

Description

The documentation can be run locally when the right packages are installed and commands are run.

Here's my understanding of the steps:

cd python
make develop
cd ..
mkdocs serve

We should document these steps, perhaps in the CONTRIBUTING.md file?

[r3stl355]

you also need to install the docs requirements first time you run it, here is the full script

(cd python; make develop)
pip install -r docs/requirements.txt
mkdocs serve

I'll add this and few other bits to CONTRIBUTING.MD

[r3stl355]

take