add re-frame/reagent playground to clojurefam resources

[tangjeff0]

https://day8.github.io/re-playground/

kudos @julionav

It's a bit more difficult to add documentation without Gitbook permissions (and even with it, because it requires some onboarding).

I was going to ask @julionav to create a PR directly, but I noticed there were a ton of duplicates folders and files in the handbook. So creating issue for this until we resolve #29