Update CONTRIBUTING documentation for the debug extension (#1460)

Author: elliette

dwds/debug_extension/CONTRIBUTING.md

@@ -11,13 +11,38 @@ pub run build_runner build web -o build -r
 ```
 pub run build_runner build web -o build
 ```
+## Local Development
+
+### Update `manifest.json`:
+
+* Change the `default_icon` in `manifest.json` to `dart_dev.png` (Note: this is not strictly necessary, but will help you to distinguish your local version of the extension from the published version)
+* [For Googlers] The developer key is needed for local development and testing. Add one of the whitelisted keys to `web/manifest.json`. IMPORTANT: DO NOT COMMIT THE KEY.
+
+```
+{
+    "name": "Dart Debug Extension",
+    "key": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
+    ...
+}
+```
+
+### Build and upload your local extension
+
+* Build the extension following the instructions above
+* Visit chrome://extensions
+* Toggle "Developer mode" on
+* Click the "Load unpacked" button
+* Select the extension directory: `/dwds/debug_extension/build/web`
+
+### Debug your local extension
+* Click the Extensions puzzle piece, and pin the Dart Debug Extension with the dev icon (unpin the published version so you don't confuse them)
+* You can now use the extension normally by clicking it when a local Dart web application has loaded in a Chrome tab
+* To debug, visit chrome://extensions and click "Inspect view on background page" to open Chrome DevTools for the extension 
+* More debugging information can be found in the [Chrome Developers documentation](https://developer.chrome.com/docs/extensions/mv3/devguide/)
+
 
 ## Deployment
 
 - Update the version number in `web/manifest.json`.
 - Zip `web/manifest.json`, `web/background.js`, and `dart.png`.
 - Upload the `.zip` to the Chrome Web Store.
-
-## [For Googlers] Local Development
-
-The developer key is needed for local development and testing. Add one of the whitelisted keys to `web/manifest.json`.