Document mergify and issue before PR

Author: ravwojdyla

docs/contributing.rst

@@ -257,5 +257,12 @@ without warnings. You can do that locally with::
 Review process
 --------------
 
-Pull requests will be reviewed by a project maintainer. All changes to *sgkit* require
-approval by at least one maintainer.
+All pull requests require pre-existing issue. If you plan to submit a PR,
+please create an issue first to describe and discuss your idea/plan/problem.
+
+Pull requests will be reviewed by a project maintainer. All changes to *sgkit*
+require approval by at least one maintainer.
+
+We use `mergify <https://mergify.io/>`_ to automate PR flow, should you like
+an automated merge of your PR, label it with ``auto-merge``, and when your PR
+gets: at least one +1 and a clean build it will get merged automatically.