add a specialisation course for deep learning from coursera (#4164)

free-courses-en.md

@@ -172,6 +172,7 @@
 
 * [Convolutional Neural Networks for Visual Recognition](http://cs231n.github.io)
 * [Deep Learning for Natural Language Processing](http://cs224d.stanford.edu)
+* [Deep Learning Specialization](https://www.coursera.org/specializations/deep-learning) (coursera)
 * [Intro to Deep Learning](https://www.kaggle.com/learn/intro-to-deep-learning) - Ryan Holbrook
 * [MIT 6.S094: Deep Learning for Self-Driving Cars](https://selfdrivingcars.mit.edu)
 * [Practical Deep Learning For Coders taught](http://www.fast.ai) - Jeremy Howard