Document the primes module.

README.md

@@ -239,6 +239,19 @@ Returns the period from the list of periods ```ps``` the given month
 
 ### Primes
 
+A very simple module for generating and checking 4-digit prime numbers.
+
+    (is-4digit-prime? n)
+
+* ```n``` - a number
+
+Returns true if the given number ```n``` has four digits and it is a
+prime number.
+
+    (gen-all-4digit-primes)
+
+Returns a list of all 4-digit prime numbers.
+
 Generic Support Modules
 -----------------------
 

primes.scm

@@ -28,9 +28,9 @@
 (module
  primes
  (
-  primes-tests!
   is-4digit-prime?
   gen-all-4digit-primes
+  primes-tests!
   )
 
  (import scheme