Add "Programming Language Foundations in Agda" (#3078)

* Add "Programming Language Foundations in Agda"

New book out from Philip Wadler and Wen Kokke.

Site: https://plfa.github.io
Repo: https://github.com/plfa/plfa.github.io

* Add authors' names

* remove trailing slash, add full names

free-programming-books.md

@@ -784,6 +784,7 @@ Kerridge (PDF) (email address *requested*, not required)
 ### Agda
 
 * [Agda Tutorial](http://people.inf.elte.hu/divip/AgdaTutorial/Index.html)
+* [Programming Language Foundations in Agda](https://plfa.github.io) - Philip Wadler and Wen Kokke
 
 
 ### Alef