misc/android: add more information to README

Add more information to misc/android/README for developing
arm and arm64 with an Android environment.

Change-Id: I0c88996b6ab0c41946a2c7e69e9c92ec7bb3be27
Reviewed-on: https://go-review.googlesource.com/112276Reviewed-by: Elias Naur <elias.naur@gmail.com>

misc/android/README

@@ -8,4 +8,17 @@ mobile subrepository:
 
 To run the standard library tests, see androidtest.bash. Run it as
 
-	CC_FOR_TARGET=.../ndk-gcc GOARCH=arm GOARM=7 ./androidtest.bash
+	CC_FOR_TARGET=$STANDALONE_NDK_PATH/bin/clang GOARCH=arm64 ./androidtest.bash
+
+To create a standalone android NDK tool chain, follow the instructions on
+
+	https://developer.android.com/ndk/guides/standalone_toolchain
+
+To run tests on the Android device, add the bin directory to PATH so the
+go tool can find the go_android_$GOARCH_exec wrapper generated by
+androidtest.bash. Then, use the same GOARCH as when androidtest.bash ran
+and set GOOS to android. For example, to run the go1 benchmarks
+
+	export PATH=$GOROOT/bin:$PATH
+	cd $GOROOT/test/bench/go1/
+	GOOS=android GOARCH=arm64 go test -bench=. -count=N -timeout=T
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