Update README to be clearer

README.md

diff --git a/README.md b/README.md
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--- a/README.md
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@@ -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).