Work on month documentation.

README.md

@@ -96,6 +96,43 @@ Specific Support Modules
 
 ### Month
 
+Module for handling months algebra to be used in period construction
+and matching.
+
+    (make-month y m)
+
+* ```y``` - a number representing valid year
+* ```m``` - a number between 1 and 12 inclusive
+
+Constructs a new month value with ```y``` number as the year component
+and ```m``` number as the month component.
+
+    (month-valid? m)
+
+* ```m``` - constructed month value
+
+Checks whether given value is structurally valid month value, the year
+is between 1000 and 9999 inclusive and the month is between 1 and 12
+inclusive. Returns boolean value.
+
+    (string->month s)
+
+* ```s``` - a string in "YYYY-MM" format
+
+Parses given string ```s``` as month and constructs a month value from
+the YYYY and MM components parsed. The resulting month value is
+returned only if it is valid. Otherwise #f is returned.
+
+    (month->string m)
+
+* ```m``` - valid month value or ```#f```
+
+Converts given month value to a string in ```"YYYY-MM"``` format.
+
+If ```#f```, returns a special empty month result ```"____-__"```.
+
+Raises an error if ```m``` is not a valid month.
+
 ### Period
 
 ### Primes