book store: Explain importance of greediness vs non-greediness (#550)

* book-store: Explain importance of 4+4 vs 5+3 case

You cannot greedily add books to a single group in this case. You have
to try the possibilities and see that two groups of four is a cheaper
grouping than a group of five and a group of three.

If this is not made clear, people may make assumptions that greedy is
the correct way to go, as in:
exercism/csharp#181

* book-store: Contrast 4+4 < 5+3 case with a 4+2 < 3+3 case

In the former, you MUST NOT be greedy.
In the latter, you MUST be greedy.

exercises/book-store/canonical-data.json

@@ -52,11 +52,17 @@
            "expected": 30.00
          },
          {
-           "description": "Two each of first 3 books and 1 copy each of rest",
+           "description": "Two groups of four is cheaper than group of five plus group of three",
            "basket": [1,1,2,2,3,3,4,5],
            "targetgrouping": [[1,2,3,4],[1,2,3,5]],
            "expected": 51.20
          },
+         {
+           "description": "Group of four plus group of two is cheaper than two groups of three",
+           "basket": [1,1,2,2,3,4],
+           "targetgrouping": [[1,2,3,4],[1,2]],
+           "expected": 40.8
+         },
          {
            "description": "Two each of first 4 books and 1 copy each of rest",
            "basket": [1,1,2,2,3,3,4,4,5],