[doc] #221: Document g1 monetary license updating process

CONTRIBUTING.md

@@ -181,3 +181,12 @@ The `--extra-index-url` is used to retrieve dependencies packages from the offic
 - PyPI distribution
   - test
   - stable
+
+### Ğ1 monetary license update
+To modify the Ğ1 monetary license files, please change them on [its repository](https://git.duniter.org/documents/g1_monetary_license), since it’s integrated in silkaj repository as a `git subtree`.
+
+To pull changes from upstream with `git subtree`, add the repository as a remote then pull:
+```bash
+git remote add g1_monetary_license https://git.duniter.org/documents/g1_monetary_license.git
+git subtree pull --prefix g1_monetary_license g1_monetary_license master
+```