Docs: Add links to sources and instructions for working on a local environment.

[hamishwillee]

So that people can work against the android SDK and develop purely on development machine if they like.

You can develop on a virtual device. To get 3DR Services on it, a developer could build it from sources if he/she's so inclined, but the simplest way is to grab the built apk (Android binary file) from the releases page, and load it on the virtual device by using Android adb toolchain and running the following command in a terminal:

adb install [.apk path].

[hamishwillee]

Add links to release page alongside the description to load the built apks onto their devices, as well as links to the sources for those so inclined to modify and build it. We have some documentation for the build and setup process in the Tower wiki that we can use as base.

And to answer you query, if you're building from source, then it would be done separately from building the app.