Merge pull request #13004 from jsquyres/pr/gh-contributors-image

docs: add image of all Open MPI github contributors

Author: jsquyres

docs/index.rst

@@ -82,3 +82,11 @@ Table of contents
    history
    man-openmpi/index
    man-openshmem/index
+
+Contributors
+============
+
+A gigantic "thank you!" to all of our contributors:
+
+.. image:: https://contrib.rocks/image?repo=open-mpi/ompi&max=999
+   :target: https://github.com/open-mpi/ompi/graphs/contributors