go.mobile: update readme, add build instructions

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/109480043

README

@@ -1,16 +1,20 @@
-This is an empty Go subrepository.  To create a new subrepository,
-visit http://code.google.com/p/go/adminSource and under repositories,
-type the new name, check [x] Clone contents and select [empty] as
-the one you want to clone.
+The go.mobile repository holds libraries and build tools for Go on Android.
 
-Then execute:
-	go get code.google.com/p/go.newrepo
-	cd $(go list -e -f '{{.Dir}}' code.google.com/p/go.newrepo)
+This is early stage work. The build system integration is not ready.
+For now,
 
-The go get will complain about not finding source code, but it will
-successfully check out the repository.
+1. Install Android Studio.
+2. Install the Android NDK.
+3. Pick a home for the standalone NDK toolchain and set an environment variable:
+	NDK_ROOT=$HOME/android/ndk-toolchain
+4. Confiure the NDK toolchain:
+	ndk/build/tools/make-standalone-toolchain.sh \
+		--platform=android-9 --install-dir=$NDK_ROOT
+5. Create an android App in studio, then set
+	ANDROID_APP=$HOME/android/studio/yourapp/app
 
-Edit the README (this file) to describe the new subrepository, and then
-use the usual hg change, mail, submit to send in the change.
-You will need to follow http://golang.org/doc/contribute.html#Code_review
-to enable the Code Review extension (pointing into your Go root).
+With these two environment variables, the make.bash script in
+go.mobile/example/hello_jni builds a .so file and symlinks in
+the necessary java files to the project.
+
+To submit changes to this repository, see http://golang.org/doc/contribute.html.