Add "Effective Go"

free-programming-books.md

@@ -912,6 +912,7 @@
 * [An Introduction to Programming in Go](http://www.golang-book.com/)
 * [Building Web Apps with Go](http://codegangsta.gitbooks.io/building-web-apps-with-go/content/)
 * [Build Web Application with Golang](https://docs.google.com/file/d/0B2GBHFyTK2N8TzM4dEtIWjBJdEk/)
+* [Effective Go](https://golang.org/doc/effective_go.html)
 * [Go Bootcamp](http://www.golangbootcamp.com/book) - Matt Aimonetti
 * [Go by Example](https://gobyexample.com/)
 * [Learning Go](http://archive.miek.nl/projects/learninggo/index.html)