accumulate: point readers to test suite

There was some confusion about whether or not the order in the README has anything to do with
the order of arguments expected in the test suite.

This is not the case, since different languages have different idioms when it comes to
enumerating. See #67.

accumulate.md

@@ -10,6 +10,8 @@ Your code should be able to produce the collection of squares:
 
 - 1, 4, 9, 16, 25
 
+Check out the test suite to see the expected function signature.
+
 ## Restrictions
 
 Keep your hands off that collect/map/fmap/whatchamacallit functionality