Update free-courses-en.md (#4516)

* Update free-courses-en.md

Added Flexbox & CSS Grid free video resources to the repo

* Update free-courses-en.md

* Update free-courses-en.md

Removed the trailing slashes from the root domain

* Update free-courses-en.md

Added free Javascript30 course from Wesbos

* Markdown video course

Added free Markdown video course

* Update free-courses-en.md

free-courses-en.md

@@ -36,6 +36,7 @@
 * [Linux](#linux)
 * [Lua](#lua)
 * [Machine Learning](#machine-learning)
+* [Markdown](#markdown)
 * [Matlab](#matlab)
 * [Mercurial](#mercurial)
 * [Misc](#misc)
@@ -382,6 +383,11 @@
 * [Stanford University Machine Learning](https://www.coursera.org/learn/machine-learning)
 
 
+### Markdown
+
+* [MasteringMarkdown](https://masteringmarkdown.com) - Wesbos
+
+
 ### MATLAB
 
 * [Data Processing and Feature Engineering with MATLAB](https://www.coursera.org/learn/feature-engineering-matlab) (coursera)