docs: Include information about the Flame DevTools extension in examp…

…le readme (#3288)

Updates a README file.

packages/flame/example/README.md

@@ -3,3 +3,35 @@
 A sample Flame game showcasing the basic game structure and the use of PositionComponents.
 
 There are a lot more more [examples](../doc/examples) and [docs](../doc).
+
+
+## Running the sample app
+
+Before running the sample app, be sure you have run `melos bootstrap` from the root
+of the `flame` repository.
+
+Then, run the sample app from your IDE or by running `flutter run` from this directory.
+
+
+## Try using the Flame developer tools when running this app
+
+`package:flame` provides developer tooling as a Flutter DevTools extension. When you
+are running an app that depends on `package:flame` from Pub, you will see the Flame
+DevTools extension automatically in Flutter DevTools and within your Flutter-supported
+IDE.
+
+**However, if you'd like to try the Flame developer tools while running this sample
+app, you will need to take one additional step.** Because the sample app depends
+on `package:flame` from source (not Pub), the Flame DevTools extension assets will
+need to be generated manually. These are normally generated on publish of the `flame`
+package, but they are not checked into source control on Github.
+
+To generate the assets for the Flame DevTools extension, run the following from the
+root of the `flame` repository:
+
+```sh
+melos devtools-build
+```
+
+Now, when you run this sample app, you can open Flutter DevTools in your IDE or in
+your browser to try out the Flame developer tools.