Add google doc add-on in readme

Signed-off-by: Jay Wang <[email protected]>

README.md

@@ -70,6 +70,8 @@ Wordflow is a social and customizable AI writing assistant tool! With Wordflow,
 
 ## Get Started
 
+### Standalone Version
+
 To use Wordflow, visit: <https://poloclub.github.io/wordflow/>.
 
 If you use macOS, we highly recommend you [creating a Web App using Safari](https://support.apple.com/en-us/104996). Open [Wordflow](https://poloclub.github.io/wordflow/) in Safari, and then from the menu bar, choose `File` > `Add to Dock`.
@@ -78,6 +80,10 @@ Note that currently [Safari doesn't support WebGPU](https://caniuse.com/webgpu).
 
 <img height="100px" src="https://i.imgur.com/hkGw5zx.png">
 
+### Google Doc Add-on
+
+If you use Google Doc, we also provide a Wordflow Add-on that you can install from the [Google Workspace Marketplace](https://workspace.google.com/marketplace/app/wordflow/851135867974). The source code of the add-on is in [this repository](https://github.com/poloclub/wordflow-doc).
+
 ## Developing Wordflow
 
 Clone or download this repository: