useful skills - adding git tutorials

docs/basics/skills.md

@@ -33,6 +33,7 @@ accomplishing a particular development goal, certainly vary.
 -   **Git** - a version control system for tracking changes to source code
     -   [Official documentation](https://git-scm.com/documentation)
     -   [15 minute interactive tutorial](https://try.github.io/levels/1/challenges/1)
+    -   [Another site with more interactive tutorials](http://learngitbranching.js.org/)
 -   **Command line / bash** - in general, "the command line" refers to using a
     text-only interface to run programs such as `civix`, `git`, and many more.
     Bash is the most common "shell" program used to execute these commands on