desktop: Update README with instructions for using non-release versions

This documents the use of BYPASS_CLI_SETUP to allow desktop development
to use the version of Boundary manually placed in
ui/desktop/electron-app/cli rather than downloading a release version.

ui/desktop/README.md

@@ -66,12 +66,23 @@ 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
+### Developing Using Non-Release Versions of Boundary
+
+You can also develop using a non-release version of Boundary - bypassing the
+above CLI download behavior - by manually placing the version of Boundary that
+you want to develop with in the `electron-app/cli/` directly (you may need to
+create the directory).
+
+After doing this, run yarn with `BYPASS_CLI_SETUP=true`; example: 
+`BYPASS_CLI_SETUP=true yarn start:desktop`.
+
+### Environment Variables
 
 These environment variables may be used to customized the build.
 
 | Variable | Description |
 | -------- | ----------- |
+| `BYPASS_CLI_SETUP` | Set to `true` to launch without bootstrapping the CLI (see above). |
 | `DEBUG_APP_UPDATER` | Enable to debug app updater feature. Must be enabled for all `APP_UPDATER_*` variables to be used. |
 | `APP_UPDATER_CURRENT_VERSION` | Version of client. |
 | `APP_UPDATER_LATEST_VERSION_TAG` | Next version for comparison with current version. |
@@ -95,6 +106,11 @@ The static production assets are saved into the `dist/` folder.
 The Boundary CLI is downloaded and extracted to `electron-app/cli/` folder as part of
 packaging. CLI version is defined in `electron-app/config/cli.js`.
 
+Similar to running in development, you can also use `BYPASS_CLI_SETUP=true` to
+bypass the download of the CLI, which can be useful for pre-release testing. See
+[Developing Using Non-Release Versions of
+Boundary](#developing-using-non-release-versions-of-boundary) for more details.
+
 #### Environment Variables
 
 These environment variables may be used to customized the build.
@@ -103,6 +119,7 @@ These environment variables may be used to customized the build.
 | -------- | ------------- | ----------- |
 | `APP_NAME` | Application Name | The user-facing name of the application, appearing in titles, etc. |
 | `BOUNDARY_DESKTOP_SIGNING_IDENTITY` | | The name of the certificate to use when signing (e.g. Developer ID Application: * (*)). |
+| `BYPASS_CLI_SETUP` | Set to `true` to launch without bootstrapping the CLI (see above). |
 
 ### Running Tests