Merge pull request #1368 from bharadwaj6/master

Added discover flask from Real Python to web development courses list.

free-courses-en.md

@@ -141,3 +141,4 @@
 ### Web Development
 * [The Odin Project - Learn Web Development for Free](http://www.theodinproject.com/)
 * [Udacity Web Development Course Catalog](https://www.udacity.com/courses#!/web-development)
+* [Discover Flask - Full Stack Web Development with Flask](https://github.com/realpython/discover-flask)