Document period lookups.

doc/calendar.md

@@ -188,3 +188,19 @@ periods ```ps```. The periods are represented as
 Returns the period from the list of periods ```ps``` the given month
 ```m``` falls into. If no period matches, returns ```#f```.
 
+    (make-period-lookup-table source)
+
+* ```source``` a list of specifications
+
+Creates a lookup table containing periods where each specification
+starts with a month and the rest of the list is value to be stored in
+the lookup table. The resulting lookup table contains periods with the
+last period being open-ended without ending month.
+
+    (lookup-by-period table)
+
+* ```table``` - period lookup table
+
+Looks up the value(s) as specified by the table created by
+```make-period-lookup-table``` for current month from parameter
+```(*current-month*)```.