Add database docs

[hdoupe]

Goals for this PR:

• Add instructions for setting up a local Postgres server
• Add instructions for running a local instance of PolicyBrain using the local Postgres server
• Add some database migrations best practice-type procedures in an effort to minimize the number of database misadventures

[GoFroggyRun]

+1 This PR LGTM.

I was able to get Postgres server set-up on my local following the instructions.

[hdoupe]

@GoFroggyRun would you mind reviewing the database back up procedure that I outlined in the DATABASE.md file?

@Kpinkelman Thanks for the notes on the local deployment setup documentation! Please, let me know if you have any more questions or feedback

[GoFroggyRun]

LGTM, +1

[hdoupe]

Thanks for the review @GoFroggyRun.