add "Go 101" book (#2894)

free-programming-books.md

@@ -1194,6 +1194,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [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/)
 * [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
 * [Go by Example](https://gobyexample.com)
 * [Go for Javascript Developers](https://github.com/bulim/go-for-javascript-developers)