Document pytest test running instruction, ability to run the tests on a dev machine

[kornpow]

Trying to run the tests for the code, I noticed that it isn't specifically documented how to run the tests.

At the moment, it also seems like the only way to run the tests is if you are on a Raspberry Pi, because of the PiCamera dep. That could be documented.

Things that might be useful:

1. Create a requirements-dev.txt which contains pytest and and other tools we may want to use. black has been suggested at times, though controversial, flake8 could be good as well.
2. Even on the RPi, and have manually installed pytest I still need to do this to run the tests PYTHONPATH=src pytest

I might try and tackle parts of this, but wanted to create an issue to remind me.