book store: Explain importance of greediness vs non-greediness (#550)
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:
https://github.com/exercism/xcsharp/issues/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.

jmluy/x-common,rpottsoh/x-common,rpottsoh/x-common,petertseng/x-common,jmluy/x-common,exercism/x-common,ErikSchierboom/x-common,kgengler/x-common,kgengler/x-common,jmluy/x-common,ErikSchierboom/x-common,Vankog/problem-specifications,exercism/x-common,Vankog/problem-specifications,petertseng/x-common