Added readme for on device unit tests

testapps/on_device_unit_tests/README.rst

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+On device unit tests
+====================
+
+This test app runs a set of unit tests, to help confirm that the
+python-for-android build is actually working properly.
+
+The main tests are for the recipes built in the apk. Each module (or
+other tool) is at least imported and subject to some basic check.
+
+This app is experimental, it doesn't yet support things like testing
+only the requirements you ask for (so if you build with requirements
+other than those specified, the tests may fail). It also has no gui
+yet, the results must be checked via logcat.
+
+Building the app
+================
+
+This app should be built using buildozer, which it also serves as a
+test for::
+
+  $ buildozer android debug
+
+Install on an Android device::
+
+  $ adb install -r adb install -r bin/p4aunittests-0.1-debug.apk
+    # or
+  $ buildozer android deploy
+
+Run the app and check in logcat that all the tests pass::
+
+  $ adb logcat | grep python  # or look up the adb syntax for this