docs: update README to include installation instructions for Gemini CLI

Author: jpoehnelt

README.md

@@ -5,3 +5,11 @@ This repository is a monorepo for tools that help Google Workspace developers.
 ## Packages
 
 - [`packages/vscode-extension`](./packages/vscode-extension): A VS Code extension with tools to help Google Workspace developers available at [Visual Studio Marketplace](https://marketplace.visualstudio.com/items?itemName=google-workspace.google-workspace-developer-tools) and [Open VSX Registry](https://open-vsx.org/extension/google-workspace/google-workspace-developer-tools).
+
+### With the Gemini CLI
+
+If you have the `gemini` command-line tool installed, you can install the extension with this command:
+
+```sh
+gemini extensions install "https://github.com/googleworkspace/developer-tools"
+```