doc: add screenshot to README

Author: ammario

README.md

@@ -34,14 +34,8 @@ of cmux as the IDE of the future. We go beyond Claude Code with:
   - LaTeX
   - Toggles!
 
-## Documentation
-
 📚 **[Read the full documentation →](https://cmux.io)**
 
-- [Installation](https://cmux.io/install.html)
-- [Keyboard Shortcuts](https://cmux.io/keybinds.html)
-- [Developer Guide](https://cmux.io/AGENTS.html)
-
 ## Quick Install
 
 Download pre-built binaries from [the releases page](https://github.com/coder/cmux/releases):
@@ -50,6 +44,12 @@ Download pre-built binaries from [the releases page](https://github.com/coder/cm
 - **Linux**: AppImage
 - **Windows**: not implemented, coming soon
 
+## Screenshots
+
+![Screenshot of mermaid diagram](./docs/img/plan-mermaid.webp)
+
+> Mermaid diagrams make it easier to review complex proposals from the Agent
+
 ## Development
 
 See [AGENTS.md](./AGENTS.md) for development setup and guidelines.