Finish month module documentation.

README.md

@@ -172,9 +172,22 @@ after ```n``` in the calendar or they are ```equal?```.
 Returns ```#t``` if both month values are valud and ```m``` comes
 after ```n``` in the calendar.
 
-month-diff
+    (month-diff f t)
 
-month-add
+* ```f``` - valid month (from)
+* ```t``` - valid month (to)
+
+Returns the difference in months from month ```f``` to month
+```t```. If both months are the same, the result is zero. If ```t```
+is before ```f```, the result is negative.
+
+    (month-add m n)
+
+* ```m``` - valid month
+* ```n``` - an integer
+
+Returns a new valid month that comes ```n``` months after ```m```. If
+```n``` is negative, it correctly subtracts the months.
 
 ### Period