Add link to Go book (#5338)

* Add link to Go book

* Link Darker Corners of Go article instead of book

books/free-programming-books.md

@@ -1488,6 +1488,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [An Introduction to Programming in Go](http://www.golang-book.com)
 * [Build Web Application with Golang](https://astaxie.gitbooks.io/build-web-application-with-golang/content/en/)
 * [Building Web Apps with Go](https://codegangsta.gitbooks.io/building-web-apps-with-go/content/)
+* [Darker Corners of Go](https://rytisbiel.com/2021/03/06/darker-corners-of-go/) - Rytis Bieliunas
 * [Effective Go](https://golang.org/doc/effective_go.html)
 * [Go 101](https://go101.org/article/101.html) - [@TapirLiu](https://twitter.com/TapirLiu)
 * [Go Bootcamp](http://www.golangbootcamp.com/book) - Matt Aimonetti