Add initial README.md for package:devtools_extensions (#6255)

There are a few TODOs in here that I will address as we continue to iterate on this package.

packages/devtools_extensions/README.md

@@ -1,2 +1,174 @@
-## devtools_extensions
-TODO: write documentation. This package is under construction.
+# Dart & Flutter DevTools Extensions
+Note: this package is under active development; more thorough documentation coming soon.
+
+Extend Dart & Flutter's developer tool suite,
+[Dart DevTools](https://docs.flutter.dev/tools/devtools/overview), with a custom tool for
+your package. DevTools' extension framework allows you to build a tool for your Dart package
+that can leverage existing frameworks and utilities from DevTools (VM service connection, theming,
+shared widgets, utilities, etc.). When an app is connected to DevTools that depends on your
+package, your extension will show up in its own DevTools tab:
+
+![Example devtools extension](_readme_images/example_devtools_extension.png)
+
+## Setup your package to provide a DevTools extension
+
+DevTools extensions must be written as Flutter web apps. This is because DevTools embeds
+extensions in an iFrame to display them dynamically in DevTools.
+
+To add an extension to your Dart package, add a top-level `extension` directory:
+```
+foo_package
+  extension/
+  lib/
+  ...
+```
+
+Under this directory, create the following structure:
+```
+extension
+  devtools/
+    build/
+    config.json
+```
+
+The `config.json` file contains metadata that DevTools needs in order to load the
+extension. Copy the `config.json` file below and fill in the approproate value for each key.
+The "materialIconCodePoint" field should correspond to the codepoint value of an icon from
+[material/icons.dart](https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/icons.dart).
+```json
+{
+    "name": "foo_package",
+    "issueTracker": "<link_to_your_issue_tracker.com>",
+    "version": "0.0.1",
+    "materialIconCodePoint": "0xe0b1"
+}
+```
+
+Now it is time to build your extension.
+
+## Build a DevTools extension
+
+### Where to put your source code
+
+Only the pre-compiled output of your extension needs to be shipped with your pub package
+in order for DevTools to load it. To keep the size of your pub package small, we recommend that
+you develop your DevTools extension outside of your pub package. Here is the recommended package structure:
+
+```
+foo_package/  # formerly the repository root of your pub package
+  packages/
+    foo_package/  # your pub package
+      extension/
+        devtools/
+          build/
+            ...  # pre-compiled output of foo_package_devtools_extension
+          config.json
+    foo_package_devtools_extension/  # source code for your extension
+```
+
+### Create the extension web app
+
+From the directory where you want your extension source code to live, run the following command,
+replacing `foo_package_devtools_extension` with `<your_package_name>_devtools_extension``:
+```sh
+flutter create --template app --platforms web foo_package_devtools_extension
+```
+
+In `foo_package_devtools_extension/pubspec.yaml`, add a dependency on `devtools_extensions`:
+```yaml
+devtools_extensions: ^1.0.0
+```
+
+In `lib/main.dart`, place a `DevToolsExtension` widget at the root of your app:
+```dart
+import 'package:devtools_extensions/devtools_extensions.dart';
+
+void main() {
+  runApp(const FooPackageDevToolsExtension());
+}
+
+class FooPackageDevToolsExtension extends StatelessWidget {
+  const FooPackageDevToolsExtension({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const DevToolsExtension(
+      child: FooDevToolsExtension(),
+    );
+  }
+}
+```
+
+The `DevToolsExtension` widget automatically performs all extension initialization required
+to interact with DevTools. From anywhere your extension web app, you can access the globals
+`extensionManager` and `serviceManager` to send messages and interact with the connected app.
+
+#### Utilize helper packages
+
+Use [package:devtools_app_shared](https://pub.dev/packages/devtools_app_shared) for access to
+service managers, common widgets, DevTools theming, utilities, and more.
+
+TODO(kenz): link examples.
+
+### Debug the extension web app
+
+#### Use the Simulated DevTools Environment (recommended for development)
+
+For debugging purposes, you will likely want to use the "simulated DevTools environment". This
+is a simulated environment that allows you to build your extension without having to develop it
+as an embedded iFrame in DevTools. The simulated environment is enabled by an environment
+parameter `use_simulated_environment`.
+
+![Simulated devtools environment](_readme_images/simulated_devtools_environment.png)
+
+To run your extension web app with this flag enabled, add a configuration to your `launch.json`
+file in VS code:
+```json
+{
+    ...
+    "configurations": [
+        ...
+        {
+            "name": "foo_devtools_extension + simulated environment",
+            "program": "foo_package/extension/foo_devtools_extension/lib/main.dart",
+            "request": "launch",
+            "type": "dart",
+            "args": [
+                "--dart-define=use_simulated_environment=true"
+            ],
+        },
+    ]
+}
+```
+
+or launch your app from the command line with the added flag:
+```sh
+flutter run -d chrome --dart-define=use_simulated_environment=true
+```
+
+#### Use a real DevTools Environment
+
+To use a real DevTools environment, you will need to perform a series of setup steps:
+
+1. Develop your extension to a point where you are ready to test your changes in a
+real DevTools environment. Build your flutter web app and copy the built assets from
+`your_extension_web_app/build` to your pub package's `extension/devtools/build` directory.
+
+> Use the `build_helper.sh` script to help with this step. (TODO(kenz): create and add link to helper script).
+
+2. Prepare and run a test application that depends on your pub package. You'll need to change the
+`pubspec.yaml` dependency to be a `path` dependency that points to your local pub package
+source code. Once you have done this, run `pub get`, and run the application.
+
+3. Start DevTools:
+    * **If you need local or unreleased changes from DevTools**, you'll need to build and run DevTools
+    from source. See `<TODO: insert instructions>` for a guide on how to do this.
+    * **If not, and if your local Dart or Flutter SDK version is >= `<TODO: insert version>`**,
+    you can launch the DevTools instance that was just started by running your app (either from
+    a url printed to command line or from the IDE where you ran your test app). You can also run
+    `dart devtools` from the command line.
+
+4. Connect your test app to DevTools if it is not connected already, and you should see a tab
+in the DevTools app bar for your extension. The enabled or disabled state of your extension is
+managed by DevTools, which is exposed from an "Extensions" menu in DevTools, available from the
+action buttons in the upper right corner of the screen.