Add courses to both HOWTO.md and free-courses-en.md (#5102)

* Add Markdown, check_urls=HOWTO.md

* Add Git course, check_urls=free-courses-en.md

HOWTO.md

@@ -6,6 +6,7 @@ Welcome to Free-Programming-Books! We welcome new contributors; even those makin
 * [Github Hello World](https://guides.github.com/activities/hello-world/)
 * [Youtube - Github Tutorial For Beginners](https://www.youtube.com/watch?v=0fKg7e37bQE)
 * [Youtube - How To Fork A GitHub Repo and Submit A Pull Request](https://www.youtube.com/watch?v=G1I3HF4YWEw)
+* [Youtube - Markdown Crash Course](https://www.youtube.com/watch?v=HUBNt18RFbo)
 
 
 Don't hesitate to ask questions; every contributor started with a first PR. You could be our thousandth!

free-courses-en.md

@@ -283,6 +283,7 @@
 * [Git and GitHub for Poets](https://www.youtube.com/playlist?list=PLRqwX-V7Uu6ZF9C0YMKuns9sLDzK6zoiV)
 * [GitHowTo](https://githowto.com)
 * [How to Use Git and GitHub](https://www.udacity.com/course/how-to-use-git-and-github--ud775) (Udacity)
+* [Introduction to Git and GitHub](https://www.coursera.org/learn/introduction-git-github) - Google (Coursera)
 
 
 ### Go