Update README.md (#6867)

Some minor tweaks to answer questions I had when looking at this.
diff --git a/dev/devicelab/README.md b/dev/devicelab/README.md
index 2e354eb..8655301 100644
--- a/dev/devicelab/README.md
+++ b/dev/devicelab/README.md
@@ -7,8 +7,13 @@
 the tests are referred to as "tasks" in the API, but since we primarily use it
 for testing, this document refers to them as "tests".
 
-You can see the continuous build results from the master branch at
-http://go/flutter-dashboard/build.html.
+If you have access to Google's internal network, you can see the continuous build results from the master branch at
+<http://go/flutter-dashboard/build.html>. (There is currently no public view of this data, unfortunately.)
+
+# Prerequisites
+
+You must set the `ANDROID_HOME` environment variable to run tests on Android. If you
+have a local build of the Flutter engine, then you have a copy of the Android SDK at `.../engine/src/third_party/android_tools/sdk`.
 
 # Running tests locally
 
@@ -20,9 +25,12 @@
 To run a test, use option `-t` (`--task`):
 
 ```sh
+# from the .../flutter/dev/devicelab directory
 dart bin/run.dart -t {NAME_OF_TEST}
 ```
 
+You can see the test names in the `manifest.yaml` file in this directory or by looking in `bin/tasks/`. Do not include the `.dart` file extension. For example, `dart bin/run.dart -t complex_layout__start_up`.
+
 To run multiple tests, repeat option `-t` (`--task`) multiple times:
 
 ```sh
@@ -41,10 +49,12 @@
 dart bin/run.dart -s {NAME_OF_STAGE}
 ```
 
+Currently there are only two stages defined, `devicelab` and `devicelab_ios`.
+
 # Reproducing broken builds locally
 
 If a commit caused a test to fail,
-[the dashboard](http://go/flutter-dashboard/build.html) might look something
+[the dashboard](http://go/flutter-dashboard/build.html) (requires access to the Google network, sorry) might look something
 like this:
 
 ![Broken Test](images/broken-test.png)