Add link to Code School's Try Git

Anybody looking for an easy to use/follow git crash course,
can use Code School's Try Git course. The entire course is
free and online.

Signed-off-by: Felipe Balbi <balbi@kernel.org>

Author: felipebalbi

toolbox.md

@@ -12,3 +12,4 @@ permalink: /toolbox/
 - [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/)
+- [Try Git](http://trygit.codeschool.com/)