Create org-wide contributing document

Create org-wide contributing document. This is needed. These are good starting points:

- https://github.com/aboutcode-org/aboutcode/blob/96bfba0dc9496fcbb335d0aa41d1a7b815d1796f/docs/source/contributing/contrib_doc.rst
- https://github.com/aboutcode-org/vulnerablecode/blob/d1f4c74610b52cff5a506e39662ad15f0f7df1ba/docs/source/contributing.rst#L3
- https://github.com/aboutcode-org/scancode.io/blob/6e131c63126be4266746b20fff8e37505f9606d2/docs/contributing.rst#L2
- https://dejacode.readthedocs.io/en/latest/contributing.html