[WebLayer] Augment the documentation on building

Change-Id: I55de2281b96b1a141ac43d75b84677ec16ce7d67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856967Reviewed-by: Darin Fisher <darin@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708124}

weblayer/README.md

@@ -29,7 +29,18 @@ Bug tracker: [Internals>WebLayer](https://bugs.chromium.org/p/chromium/issues/li
 
 `renderer` internal code which runs in the renderer process
 
-## Testing
+## Setting Up Your Build Environment
+
+If you haven't done this already, you first need to set up an Android build. If
+you are a Google employee, reach out to weblayer-team@google.com for internal
+instructions. Otherwise follow the [Android build instructions](/docs/android_build_instructions.md).
+
+## Building and Testing
+
+To run the sample app:
+
+$ autoninja -C out/Default run_weblayer_shell
+$ out/Default/bin/run_weblayer_shell
 
 To run instrumentation tests: