Update free-programming-books.md (#3339)

* Update free-programming-books.md

Add gidti eBook

* Add author name

free-programming-books.md

@@ -67,6 +67,7 @@
   * [Bootstrap](#bootstrap)
 * [HTTP](#http)
 * [Icon](#icon)
+* [Idris](#idris)
 * [iOS](#ios)
 * [IoT](#iot)
 * [Isabelle/HOL](#isabellehol)
@@ -1369,6 +1370,11 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Introduction to HTTP](https://launchschool.com/books/http) - Launch School
 
 
+### Idris
+
+* [Gentle Introduction to Dependent Types with Idris](https://leanpub.com/gidti) - Boro Sitnikovski  *(Leanpub account or valid email requested)*
+
+
 ### Icon
 
 * [The Implementation of the Icon Programming Language](http://www.cs.arizona.edu/icon/ibsale.htm)