Clarification of branch to use and build instructions for new users

[mikew67]

Problem: With turbulent changes in this rep, it's unclear for new users/testers how to build the app and which branch to use.

Solution* Add two entries to the README for this repo:

• A pointer to "Getting Started" a build instructions doc (and might as well add testing instructions, too). (Mark Ingle has written a build doc for iOS already).
• A (dynamically changing?) note in the README abut which repo branch to build from.