Improve ReadMe with changes reflecting MySQL support

[WoosterInitiative]

Description

Current Readme is identical to the standard PostgreSQL-based https://github.com/cookiecutter/cookiecutter-django repo.

Rationale

This is both confusing (for example, it still has the link to this project in it) and not helpful.

It would be helpful for someone like me that only sets up a new environment once in a while to have instructions on how to setup my MySQL environment as it differs from PostgreSQL from the docs.