update build instructions (docker example)

BUILD.md

@@ -0,0 +1,49 @@
+# Build Instructions
+
+## Android
+
+All commands are executed in the root directory of this repo unless otherwise stated.
+
+* Follow the [Cordova install platform Guide for
+  Android](https://cordova.apache.org/docs/en/latest/guide/platforms/android/index.html)
+
+  Or instead pull an appropriate [docker image](https://hub.docker.com/). For
+  instance [beevelop/cordova](https://github.com/beevelop/docker-cordova):
+
+  ```
+  docker pull beevelop/cordova:latest
+  ```
+
+  You can then run docker in the ForRunners directory interactively:
+
+  ```
+  docker run -it --privileged -v /dev/bus/usb:/dev/bus/usb -v $PWD:/tmp beevelop/cordova /bin/bash
+  ```
+
+* Select the platform and version you want to build for (your target
+  device). It is advisable to check if all requirements for the specified
+  platform are met.
+
+  ```
+  cordova platform add android@~7.0.0
+  cordova requirements
+  ```
+
+  This installs the cordova plugins needed for this platform.
+
+* Build the app for Android.
+
+  ```
+  cordova build android
+  ```
+
+* Run on an Android device.
+
+  ```
+  cordova run android
+  ```
+
+  Be sure the debugging mode of your phone is enabled and no other adb servers
+  are running. Your phone should be listed in `adb devices`.
+
+  You may also need to uninstall the old version first.