update README.md with .json doc.
update README.md with .json doc.

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