Mention JavaScript runtime requirement

Author: ras44

readme.md

@@ -40,6 +40,7 @@ You can take all the files of this site and run them just on your computer as if
 * [Jekyll](https://jekyllrb.com/)
 * [Ruby](https://www.ruby-lang.org/en/)
 * [Git](https://git-scm.com/)
+* [A JavaScript runtime (ie. node.js)](https://github.com/rails/execjs)
 
 _If you have installed [GitHub Desktop](https://desktop.github.com), Git was also installed automatically._