[doc] #134: Document how to update the copyright year

CONTRIBUTING.md

@@ -25,6 +25,10 @@ poetry run black duniterpy/file.py
 With `pre-commit`, Black is called on staged files, so the commit should fail in case black would make changes.
 You will have to add Black changes in order to commit your changes.
 
+### Update copyright year
+Follow [this documentation](https://github.com/Lucas-C/pre-commit-hooks#removing-old-license-and-replacing-it-with-a-new-one)
+Only difference is to update the year in `license_header.txt` rather than `LICENSE.txt`.
+
 ## Release workflow
 To handle a release, you have to respect this workflow: