android: add readme

README.android.md

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+# Uxn on Android
+
+This is a working port of Uxn to run on Android.
+
+## Building yourself
+
+Easiest way to start is to install JDK first, then [Android
+Studio](https://developer.android.com/studio) and make sure NDK is
+there to use on your computer.  Your OS package manager might already
+have the packages required, if you're willing to go that route.
+
+Make sure environment is properly configured:
+
+	; ls $ANDROID_HOME
+	build-tools  emulator  licenses  ndk  patcher  platform-tools  platforms  tools
+
+You should be able to build now:
+
+	; cd uxn; ./build --no-run                 # need boot.rom to be included
+
+	; cd src/android; ./gradlew assembleDebug # now the APK
+	............
+	BUILD SUCCESSFUL in 111s
+	31 actionable tasks: 3 executed, 28 up-to-date
+
+	; find app -name *.apk
+	app/build/outputs/apk/debug/app-debug.apk
+
+Refer to Android development documentation for more details.