Added Flask Documentation book. (#4756)

* Added Flask Documentation

* Update free-programming-books.md
Co-authored-by: NSeth <seth@elypia.org>
Co-authored-by: NSeth <seth@elypia.org>

free-programming-books.md

@@ -2410,6 +2410,7 @@ Kerridge (PDF) (email address *requested*, not required)
 #### Flask
 
 * [Explore Flask](https://exploreflask.com) - Robert Picard
+* [Flask Documentation](https://flask.palletsprojects.com) - Pallets (:construction: *in process*)
 * [The Flask Mega-Tutorial](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world) - Miguel Grinberg (0.9)