Add some explanatory text intended to help contributors run tests

[onpaws]

Just a few small steps I had to figure out recently, hopefully helps future contributors save some time.

[onpaws]

Nice, thanks! Can you link to this from the main README too so that people can find it?

Good shout, added

[onpaws]

Might be nice to add a Linux-specific section in future, although it ostensibly should be easy enough to suss out based on the macOS instructions.

Please give a shout if anything needs attention, otherwise intending to hit the green button

[onpaws]

Merging