docs: ✏️ Add tips on testing to developer docs

Inform how to run copier with a dirty state.

docs/src/developer.md

@@ -93,3 +93,15 @@ If this is the first time you work with this repository, follow the instructions
    ```
 
 - Then you can open a pull request and work with the reviewer to address any issues.
+
+## Tips
+
+### Testing local changes
+
+To test you local modifications, you can run copier with the `--vcs-ref HEAD` flag and point to your local clone. This will use the latest changes, including uncommitted modifications (i.e., the dirty state).
+What I normally do is this:
+
+```bash
+cd $(mktemp -d) # Go to a tmp folder
+copier copy --vcs-ref HEAD /path/to/clone/ pkg # Clone dirty clone into pkg
+```