add ref for markdown

2021/2021-03-18_basicGitTraining/slides/essential_commands.md

@@ -45,6 +45,9 @@ $ cp firstnameLastname.md myName.md
 
 Then, make your changes with your favorite editor!
 
+<br><br>
+Reference for markdown: https://howto.lcsb.uni.lu/?contribute:markdown
+
 
 
 # Add your file to the stage