Luhn - add edge case description.
Luhn - add edge case description.

A 0 or 1 length string is not valid, and nor is a string
with non-space characters in.

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