docs: add instructions for upgrading the extension to the latest version

- Added a new section in README for upgrading the `commitmsg` extension.
- Included a sample command to guide users on performing the upgrade.

This update improves documentation by providing clear steps for users to
keep their extension up-to-date.

Author: hazadus

README.md

@@ -61,6 +61,13 @@ gh commitmsg --examples 5
 gh commitmsg --model xai/grok-3-mini
 ```
 
+### Upgrading Extension to Latest Version
+
+```bash
+# Use to upgrade commitmsg to latest version
+gh extension upgrade commitmsg
+```
+
 ### Output examples
 
 LLM will generate something like: