Update README.md: Added link to Git Tutorial IPython notebook

"Version control for fun and profit: the tool you didn't know you needed. From personal workflows to open collaboration" via http://www.reddit.com/r/IPython/comments/1gsq79/version_control_for_fun_and_profit/

... http://www.reddit.com/r/learnpython/wiki/index
Merge pull request #1 from westurner/patch-1

Update README.md: Added link to Git Tutorial IPython notebook