Add section about how you become a committer

CONTRIBUTING.rst

@@ -236,3 +236,15 @@ but here is a simple overview:
     base: features        # if it's a feature
 
 
+Joining the Development Team
+----------------------------
+
+Anyone who has successfully seen through a pull request which did not
+require any extra work from the development team to merge will
+themselves gain commit access (if we forget please send a friendly
+reminder).  This does not mean your workflow to contribute changes,
+everyone goes through the same pull-request-and-review process and
+no-one merges their own pull requests.  It however means you can
+participate in the development process more fully since you can merge
+pull requests from other contributors yourself after having reviewed
+them.