docs: 📝 explain how testing works in contributing guide (#127)

# Description

Using the `just test` can only get from committed changes, so added this
to contributing.

This PR needs a quick review.

## Checklist

- [x] Formatted Markdown
- [x] Ran `just run-all`

Author: lwjohnst86

CONTRIBUTING.md

@@ -42,3 +42,8 @@ a terminal so that the working directory is the root of this project
 ``` bash
 just run-all
 ```
+
+When testing the template, copier can only use content kept in the Git
+history. Meaning that if you make changes to the template and try to
+test it, it won't be able to test those changes. You have to commit the
+changes first in order for copier to use them.