Update README to be clearer
diff --git a/README.md b/README.md
index 74ae74e..0783981 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,10 @@
## Overview
-This repository contains the Skylark implementation of Android rules in Bazel.
+This repository is used to contain the Skylark implementation of Android rules in Bazel.
-For the list of Android binary and library rules, see the Bazel [documentation](https://docs.bazel.build/versions/master/be/android.html).
+The rules are being incrementally open-sourced from their native implementations
+in the [Bazel source
+tree](https://source.bazel.build/bazel/+/master:src/main/java/com/google/devtools/build/lib/bazel/rules/android/).
+
+For the list of Android rules, see the Bazel [documentation](https://docs.bazel.build/versions/master/be/android.html).
