docs: contribute page

{{cookiecutter.project_slug}}/CONTRIBUTE.md → {{cookiecutter.project_slug}}/CONTRIBUTING.md

@@ -5,6 +5,8 @@ We try to make it easy, and all contributions, even the smaller ones, are more t
 This includes bug reports, fixes, documentation, examples...
 But first, read this page (including the small print at the end).
 
+Contributions are available on https://gitlab.com/to-be-continuous/{{cookiecutter.project_slug}}.
+
 ## Legal
 
 All original contributions to _to be continuous_ are licensed under the