Add Web Information Retrieval video course (#3397)

free-courses-en.md

@@ -453,3 +453,4 @@
 * [Flask(A Python Microframework) Tutorial](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world)
 * [Free Code Camp](https://www.freecodecamp.org)
 * [The Odin Project - Learn Web Development for Free](http://www.theodinproject.com)
+* [Web Information Retrieval](https://www.youtube.com/playlist?list=PLAQopGWlIcya-9yzQ8c8UtPOuCv0mFZkr) - L. Becchetti, A. Vitaletti (University of Sapienza Rome)