bpo-33807: Add Python 3.7 buildbot link to '.github/CONTRIBUTING.rst' (GH-7532)

diff --git a/.github/CONTRIBUTING.rst b/.github/CONTRIBUTING.rst
index d559bd5..7f912e8 100644
--- a/.github/CONTRIBUTING.rst
+++ b/.github/CONTRIBUTING.rst
@@ -8,6 +8,10 @@
 
   + `Stable buildbots <http://buildbot.python.org/3.x.stable/>`_
 
+- 3.7
+
+  + `Stable buildbots <http://buildbot.python.org/3.7.stable/>`_
+
 - 3.6
 
   + `Stable buildbots <http://buildbot.python.org/3.6.stable/>`_