Create getting started documentation for local testing and eval

[pombredanne]

In #47 we now have a nice way to test things locally for evaluation:

• Function(s) and scripts for PurlDB to subscribe to Package updates over AP #47 (comment)

As a follow up, we should make that part of the standard doc.

Also from a private report:

Small note, SECRET_KEY needs to be set in the env file of purldb and federatedcode but wasnt in the instructions at #47 so perhaps worth adding to documentation.