Added How To Code in Go book (#4666)

free-programming-books.md

@@ -1390,6 +1390,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Go Bootcamp](http://www.golangbootcamp.com/book) - Matt Aimonetti
 * [Go by Example](https://gobyexample.com)
 * [Go for Javascript Developers](https://github.com/bulim/go-for-javascript-developers)
+* [How To Code in Go](https://www.digitalocean.com/community/books/how-to-code-in-go-ebook) - Mark Bates, Cory Lanou, and Timothy J. Raymond (PDF, EPUB)
 * [Learn Go with Tests](https://quii.gitbook.io/learn-go-with-tests/)
 * [Learning Go](https://miek.nl/go/)
 * [Let's learn Go!](http://go-book.readthedocs.io/en/latest/)