add golang-challenge

The Go Challenge is a monthly programming challenge for Go developers.

README.md

@@ -1345,6 +1345,7 @@ Where to discover new Go libraries.
 * [awesome-awesomeness](https://github.com/bayandin/awesome-awesomeness) - List of other amazingly awesome lists.
 * [Flipboard - Go Magazine](https://flipboard.com/section/the-golang-magazine-bVP7nS) - A collection of Go articles and tutorials.
 * [Go Blog](http://blog.golang.org) - The official Go blog.
+* [Go Challenge](http://golang-challenge.org/) - Learn Go by solving problems and getting feedback from Go experts.
 * [Go Forum](https://forum.golangbridge.org) - Forum to discuss Go.
 * [Go Projects](https://github.com/golang/go/wiki/Projects) - List of projects on the Go community wiki.
 * [gocryforhelp](https://github.com/ninedraft/gocryforhelp) - A collection of Go projects that needs help. Good place to start your open-source way in Go.