Use a non-debug keystore to sign official APK builds.

The keystore is in Google Storage with restricted read access. Password is in the usual internal password store.

R=ppi@chromium.org, ppi

Review URL: https://codereview.chromium.org/1183313003.
diff --git a/README.md b/README.md
index cb7a361..1fb6537 100644
--- a/README.md
+++ b/README.md
@@ -133,6 +133,12 @@
 $ ninja -C out/Debug -j 1000
 ```
 
+### Official builds
+
+Official builds for android generate a signed Mojo Shell intended for
+distribution. You normally should not need to produce one. If you have any
+questions, reach out to [etiennej@chromium.org](mailto:etiennej@chromium.org).
+
 ## Update your checkout
 
 You can update your checkout like this. The order is important. You must do the