[skip ci] Add readme generation doc link in sync exercise section (#846)
[skip ci] Add readme generation doc link in sync exercise section (#846)

of CONTRIBUTING.md

Came up during https://github.com/exercism/javascript/pull/836
exercism/xecmascript,exercism/xecmascript,tejasbubane/xecmascript,tejasbubane/xecmascript