docs: Add app updater dev env vars to readme

ui/desktop/README.md

@@ -65,6 +65,16 @@ To run as a desktop app:
 The Boundary CLI is downloaded and extracted to `electron-app/cli/` folder as part of
 build. CLI version is defined in `electron-app/config/cli.js`.
 
+#### Environment Variables
+
+These environment variables may be used to customized the build.
+
+| Variable | Description |
+| -------- | ----------- |
+| `DEBUG_APP_UPDATER` | Enable to debug app updater feature. Must be enabled for all `APP_UPDATER_*` variables to be used. |
+| `APP_UPDATER_LATEST_VERSION_TAG` | Version string for comparison with current release version. |
+| `APP_UPDATER_LATEST_VERSION_LOCATION` | Location of app release to use for updating client. Can be a filepath or url. |
+
 ### Building for Production
 
 Before executing a build, be sure to set any environment variables necessary