update contributing.md

docs/contributing.md

@@ -135,6 +135,20 @@ other people can use it as well. This section collects some technical
 information that you will need for updating Catch2's documentation, and
 possibly some generic advise as well.
 
+We use mkdocs for documentation and poetry for python environment.
+first install poetry (you'll need python...)
+```console
+pip install poetry
+```
+Poetry will install our docs dependencies
+```console
+poetry install
+```
+serve the docs locally to see your changes
+```console
+poetry run mkdocs serve
+```
+
 
 ### Technicalities