Add dev guide for building locally

[dstansby]

👋 I dropped by hoping to contribute, but have struggled to get the project building locally on my laptop as a starting point. It would be great to add some quick instructions to get potential contributors up and running with a development environment/installation. I'd be happy to test-drive anything as a fresh pair of eyes since I haven't done much android development before.

[HilaryN]

@dstansby it would be great to have your contribution.
What version of Java do you have? I remember I had problems because I was on the wrong version.
There are some instructions on the wiki but they may well be out of date :-)