difference-of-squares: update README to include formula hint

Author: bitfield

exercises/difference-of-squares/README.md

@@ -12,6 +12,10 @@ Hence the difference between the square of the sum of the first
 ten natural numbers and the sum of the squares of the first ten
 natural numbers is 3025 - 385 = 2640.
 
+You are not expected to discover an efficient solution to this yourself from
+first principles; research is allowed, indeed, encouraged. Finding the best
+algorithm for the problem is a key skill in software engineering.
+
 ## Running the tests
 
 To run the tests run the command `go test` from within the exercise directory.