docs: add slash commands section

docs/USAGE.md

@@ -25,6 +25,16 @@ tuimble --host mumble.example.com --user myname
 | `q` | Quit |
 | `Ctrl+C` | Quit |
 
+## Slash Commands
+
+| Command | Action |
+|---------|--------|
+| `/help` | List available commands |
+| `/deafen` | Toggle self-deafen |
+| `/mute` | Toggle self-mute (suppresses PTT) |
+| `/unmute` | Unmute yourself |
+| `/register` | Register your identity on the server |
+
 ## Push-to-Talk Modes
 
 - **toggle** — press to start, press again to stop (default)