Add descriptions to each exercise

hamming_distance.lua

@@ -1,3 +1,8 @@
+-- The hamming distance is the number of positions in which two
+-- strings differ, and is used to measure how similar strings are.
+-- Implement the hamming_distance() function below to calculate this;
+-- you may assume that the strings are of equal length.
+--
 -- Solution --------------------------------------------------------------------
 function hamming_distance(str1, str2)
 	-- Your implementation here