Add note to run migrate Django command

[gregoryfoster]

When stepping through the regulations-site README.md file's Quick start section, I found that I needed to run the Django migrate command prior to starting the server. That should probably be added to the docs.

python manage.py migrate

I also found the npm install command happily ran without executing the grunt task afterwards because the grunt executable was not available in my environment. Installing the grunt-cli package is well-documented further down the README, but it might be helpful to post a notice if this rare circumstance is identifiable after npm install executes.

Finally, I had to create a local_settings.py file to specify API_BASE, but that's already documented in #140.

Put it all together, and I've got a usable regulations-site instance running locally 👍 Thanks again for the team's hard work on this very exacting domain.