CONTRIBUTING: Document contribution through the Kitware GitLab Instance

Replace the instructions that routed patches to the mailing list with
new instructions routing them to our GitLab instance.

CONTRIBUTING.rst

@@ -6,22 +6,26 @@ Community
 
 CMake is maintained and supported by `Kitware`_ and developed in
 collaboration with a productive community of contributors.
+Please subscribe and post to the `CMake Developers List`_ to raise
+discussion of development topics.
 
 .. _`Kitware`: http://www.kitware.com/cmake
-
-The preferred entry point for new contributors is the mailing list.
-Please subscribe and post to the `CMake Developers List`_ to offer
-contributions.  Regular and productive contributors may be invited
-to gain direct push access.
-
 .. _`CMake Developers List`: https://cmake.org/mailman/listinfo/cmake-developers
 
 Patches
 =======
 
-Please base all new work on the ``master`` branch.  Then use
+CMake uses `Kitware's GitLab Instance`_ to manage development and code review.
-``git format-patch`` to produce patches suitable to post to
+To contribute patches:
-the mailing list.
+
+#. Fork the upstream `CMake Repository`_ into a personal account.
+#. Base all new work on the upstream ``master`` branch.
+#. Create commits making incremental, distinct, logically complete changes.
+#. Push a topic branch to a personal repository fork on GitLab.
+#. Create a GitLab Merge Request targeting the upstream ``master`` branch.
+
+.. _`Kitware's GitLab Instance`: https://gitlab.kitware.com
+.. _`CMake Repository`: https://gitlab.kitware.com/cmake/cmake
 
 Code Style
 ==========