feat: Add Plugin Button Registration API documentation and Plugin System implementation summary

- Introduced PLUGIN_BUTTON_API.md detailing button registration, UI locations, and usage examples.
- Added PLUGIN_SYSTEM_IMPLEMENTATION.md summarizing implemented features, usage examples, and API details for the plugin system.

README.md

@@ -29,3 +29,50 @@ To build the app locally, run:
 
 To start local dev server, run:
 * `npm run dev`
+
+## Plugin System
+
+Paarrot includes a plugin system for extending and customising the app with JavaScript modules.
+
+### Installing Plugins
+
+1. Drop a plugin folder into your plugins directory:
+   - **Windows**: `%APPDATA%\paarrot\plugins\<plugin-name>\`
+   - **Linux**: `~/.config/Paarrot/plugins/<plugin-name>/`
+   - **macOS**: `~/Library/Application Support/Paarrot/plugins/<plugin-name>/`
+2. Enable it in **Settings → Plugins → Installed**
+
+### What Plugins Can Do
+
+- Register custom slash commands
+- Intercept and modify messages
+- Inject buttons into 11 UI locations (nav lists, toolbars, headers, sidebar, menus)
+- Register custom themes
+- Hook into raw Matrix events
+- Run background tasks
+- Show system notifications
+
+### Plugin Locations (UI Buttons)
+
+Plugins can inject buttons as **nav list rows** or **icon buttons** across the app:
+
+| Location | Style | Where |
+|---|---|---|
+| `channel-list` | Nav row | Space channel list |
+| `home-section` | Nav row | Home panel, above room list |
+| `direct-messages` | Nav row | DMs panel, below "Create Chat" |
+| `sidebar-actions` | Icon | Left sidebar — above Explore and above Search |
+| `text-composer-toolbar` | Icon | Message composer toolbar |
+| `composer-actions` | Icon | Beside the `+` attach button |
+| `room-header` | Icon | Room header bar |
+| `room-menu` | Icon | Room ⋮ dropdown |
+| `message-actions` | Icon | Message hover bar |
+| `user-menu` | Icon | Right-click on user avatar |
+| `search-notification-section` | Icon | Notifications page header |
+
+### Documentation
+
+- [Plugin System Overview](PLUGINS.md)
+- [Full Plugin API Reference](docs/PLUGIN_API.md)
+- [Button Registration API](docs/PLUGIN_BUTTON_API.md)
+- [Example Plugins](plugins/)