added Communicating using Markdown Course (#6109)

* added Communicating using Markdown Course

* Update free-courses-en.md

* Update courses/free-courses-en.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

courses/free-courses-en.md

@@ -615,6 +615,7 @@
 
 ### Markdown
 
+* [Communicating using Markdown](https://lab.github.com/githubtraining/communicating-using-markdown) - GitHub Learning Lab *(GitHub account or email address required)*
 * [MasteringMarkdown](https://masteringmarkdown.com) - Wesbos