contribute: add note regarding building problem

Following the instructions for building the documention errors out. This
commit adds a note to aknowledge the issue and a link to the relevant
GitLab issue.

Change-Id: I030a10d8c541feafa5c93b569e2e285b6fb6c5ee

contribute.rst

@@ -124,6 +124,11 @@ From the base of the repository, run:
 You should now be able to view the documentation in your web
 browser.  The homepage is at ``_build/html/index.html``.
 
+.. note:: This documentation does not currently build with the latest
+          version of sphinx. Please see `this issue on GitLab
+          <https://git.jami.net/savoirfairelinux/jami-docs/-/issues/15>`_ 
+          for a workaround and updates regarding this problem.
+
 To automatically build the documentation and refresh your web browser whenever
 you save changes, run: