📝 Add direction to work from a branch of main repo rather than a fork

[e-lo]

Per @Danishswag's recent contribution – this needs to be made more obvious in CONTRIBUTING.md

[Danishswag]

As a follow-up to this, if we're updating CONTRIBUTING.md anyway, another issue I ran into is that the tests it recommends running also requires the packages in docs/requirements.txt too since the first test script (tests/test_local_spec) builds the docs locally.

If we want to keep the test suite as is, we could maybe just merge the two requirements files?

[e-lo]

The first requirement in requirements.txt is the docs/requirements.txt, and then the development setup instructions in CONTRIBUTING.md says:

Install development requirements packages pip install -r requirements.txt or in a virtual environment.

Is there somewhere else we should add this instruction?

[Danishswag]

Must have been an issue with my Python install, please feel free to ignore