Add data visualization course 2020 (EbookFoundation#5207)

* Add data visualization course 2020

* Sorted and removed YouTube tags

Author: yeehanchung

courses/free-courses-en.md

@@ -211,6 +211,7 @@
 * [Data Mining with Weka MOOC](https://www.cs.waikato.ac.nz/ml/weka/mooc/dataminingwithweka/)
 * [Data Science interview questions](https://www.youtube.com/playlist?list=PLZoTAELRMXVPkl7oRvzyNnyj1HS4wt2K-) - Krish Naik
 * [Data Visualization by Alexis Cook at Kaggle](https://www.kaggle.com/learn/data-visualization)
+* [Datavis 2020](https://www.youtube.com/playlist?list=PL9yYRbwpkykuK6LSMLH3bAaPpXaDUXcLV) - Curran Kelleher
 * [Introduction to Data Science in Python](https://www.coursera.org/learn/python-data-analysis) - Christopher Brooks (Coursera)
 * [Introduction to Python for Data Science](https://www.edx.org/course/introduction-python-data-science-microsoft-dat208x-3)
 * [More Data Mining with Weka MOOC](https://www.cs.waikato.ac.nz/ml/weka/mooc/moredataminingwithweka/)