Prepare period documentation and improve static dependencies.

README.md

@@ -191,6 +191,21 @@ Returns a new valid month that comes ```n``` months after ```m```. If
 
 ### Period
 
+This module implements simple calendar period handling with month
+granularity.
+
+sort-period-markers
+
+period-markers->periods
+
+periods-duration
+
+month-in-periods?
+
+periods->string
+
+periods-match
+
 ### Primes
 
 Generic Support Modules