docs(README): add Update section warning about reinstall after git pull

Clarify that `git pull` / `git submodule update` only update the source
tree; `make install` or `mytoolkit update` must be rerun for changes to
take effect in the installed CLI.

README.md

@@ -10,6 +10,15 @@ mytoolkit --version
 mytoolkit templates register ~/workspace/templates   # one-time, points at template root
 ```
 
+## Update
+
+`git pull` or `git submodule update` only updates the source tree. The installed Python package is **not** refreshed automatically. After pulling new code, rerun:
+
+```bash
+make install            # or: mytoolkit update
+mytoolkit --version     # verify the new version is active
+```
+
 ## Top-level commands
 
 ```bash