Document developer instructions

Summary:
D1034 / 70aba78f9e introduced an Arcanist configuration,
so we can document how to use it.

Basically, it's an "install rSCL shellcheck + pep8 + flake8".

Test Plan: Follow the rSCL and venv procedures documented.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1036