[#4420] Add a RetainerDeliverable model.

app/models/retainer_deliverable.rb

@@ -0,0 +1,8 @@
+# A RetainerDeliverable is an HourlyDeliverable that is renewed at
+# regular calendar periods.  The Company bills a regular number of
+# hours for a hourly rate whereby the budgets are reset over a
+# regular cyclical period (often monthly).
+class RetainerDeliverable < HourlyDeliverable
+  unloadable
+
+end