Document processes required for PRs that change the DB structure

[hughrun]

In the "Developer Environment" page there's a (very helpful!) list of commands to run to set up your environment. It may be helpful to add something somewhere about what to do if your PR is going to change the database structure. This could include:

• running docker-compose run --rm web python manage.py makemigrations before docker-compose run --rm web python manage.py migrate
• checking in your final clean migration files
• merging migrations if necessary
• preferred minimum tests ("anything that posts to a model" for example)

"Developer Environment" may not be the most sensible place for this, possibly it needs its own section I'm not sure.

[mouse-reeve]

I think it would make sense to add a separate section for pull requests