docs: contributing: add contrib page

Signed-off-by: Nicolas Froger <nico@cri.epita.fr>

docs/contributing/contributing.md

+# Contributing
+
+Most of CRI's services are available as open source free software. This means
+you can read the code that run some of our services and contribute to them. You
+can find all of our open-source code on our [Gitlab
+group](https://gitlab.cri.epita.fr/cri).
+
+Services developed by the CRI can be found under the "Services" subgroup. The
+infrastructure written in "IaC" can be found in the "Infrastructure as Code"
+subgroup.

mkdocs.yml

@@ -33,5 +33,6 @@ nav:
     - 'from_home/docker.md'
     - AFS: 'from_home/afs.md'
   - Contributing:
+    - 'contributing/contributing.md'
     - 'contributing/map.md'
     - 'contributing/doc.md'