Add local build instructions for Android to README

[seehorne]

Is your feature request related to a problem? Please describe.
While iOS has testflight, Android users currently need to build in order to install the app. Eventually, it would be best if they have an APK, but in the short term, providing instructions for how to do this build would make it more feasible for users to take on this installation.

Describe the solution you'd like

• Document process for creating the Android build locally and getting it on device
• Add instructions for this process to the README