add additional git links

Author: justmarkham

toolbox.md

@@ -8,4 +8,7 @@ permalink: /toolbox/
 
 ## Git/GitHub
 
-- [Git & GitHub Video Playlist](https://www.youtube.com/playlist?list=PL5-da3qGB5IBLMp7LtN8Nc3Efd4hJq0kD)
+- [Git & GitHub Video Playlist](https://www.youtube.com/playlist?list=PL5-da3qGB5IBLMp7LtN8Nc3Efd4hJq0kD) (also available for [download](https://drive.google.com/folderview?id=0BxRfg0msVmAoRlZFQjJ3T3VTOUE&usp=sharing) as mp4 files)
+- [A Beginner's Quick Reference Guide for Git Commands](http://www.dataschool.io/git-quick-reference-for-beginners/)
+- [Understanding the Relationship Between Git and GitHub](http://www.dataschool.io/github-is-just-dropbox-for-git/)
+- [Simple Guide to GitHub Forks](http://www.dataschool.io/simple-guide-to-forks-in-github-and-git/)