Update README.md

 - [Update() syntax](http://cran.r-project.org/doc/manuals/R-intro.html#Updating-fitted-models)
 - [Stepwise regression example](http://ww2.coastal.edu/kingw/statistics/R-tutorials/multregr.html)
 - [What does summary(lm(...)) output mean?](http://stats.stackexchange.com/questions/5135/interpretation-of-rs-lm-output)
+- [Scikit-Learn ML Cheatsheet](http://peekaboo-vision.blogspot.com.au/2013/01/machine-learning-cheat-sheet-for-scikit.html)