Add find_by_discid method.

lib/musicbrainz/models/release.rb

@@ -26,6 +26,13 @@ module MusicBrainz
           create_model: :release
         })
       end
+
+      def find_by_discid(id)
+        client.load(:discid, { id: id, inc: [:media, :release_groups] }, {
+          binding: :release_group_releases,
+          create_models: :release
+        })
+      end
     end
   end
 end