Add instructions for running tests

[ericyhwang]

The tests in this repo are integration tests that don't work without a Mongo already running, but there aren't currently any instructions, which can be confusing for first-time contributors.

Having the instructions also helps more frequent contributors like us too. It always takes me a few minutes to figure out the exact Docker invocation to start the container.

[coveralls]

Coverage remained the same at 93.458% when pulling abc7a25 on test-instructions into ea464d9 on master.

[coveralls]

Coverage remained the same at 93.458% when pulling abc7a25 on test-instructions into ea464d9 on master.

[alecgibson]

This looks fine to me, although my immediate thoughts are:

• not everyone has Docker installed (and it's a pretty heavy install)
• perhaps we should move to using something like mongodb-memory-server? We're using it on one of our projects, and it seems to work pretty well. It's nice because it'll "just work" regardless of development setup