Adding local celery instructions to developing-locally

[pjhoberman]

Description

Updating documentation to make running celery locally easier.

Checklist:

• [x ] I've made sure that tests/test_cookiecutter_generation.py is updated accordingly (especially if adding or updating a template option)
• [ x] I've updated the documentation or confirm that my change doesn't require any updates

Rationale

I personally spent a lot of time and tabs trying to find this answer, and thought I could save others time with this small addition.

[browniebroke]

Thanks!