Update contributing docs to describe process for submitting PR to add plugin to list.

Fixes #2093. /cc @mattr-

site/docs/contributing.md

@@ -106,6 +106,11 @@ The [Jekyll wiki]({{ site.repository }}/wiki) on GitHub
 can be freely updated without a pull request as all 
 GitHub users have access.
 
+If you want to add your plugin to the [list of plugins](/docs/plugins/#available_plugins),
+please submit a pull request modifying the [plugins page source
+file]({{ site.repository }}/blob/master/site/docs/plugins.md) by adding a
+link to your plugin under the proper subheading depending upon its type.
+
 Gotchas
 -------