Merge pull request DataScienceSpecialization#8 from justmarkham/updates

Updates to About, Practical Machine Learning, and Data Scientist's Toolbox

Author: seankross

about.md

@@ -16,5 +16,5 @@ The [Data Science Specialization](https://www.coursera.org/specialization/jhudat
 
 ### Community Contributors:
 
-- Kevin Markham
+- [Kevin Markham](http://www.dataschool.io/)
 - Derek Franks

pml.md

@@ -4,3 +4,6 @@ title: Practical Machine Learning
 permalink: /pml/
 ---
 
+## Model Evaluation
+
+- [Simple Guide to Confusion Matrix Terminology (sensitivity, specificity, etc.)](http://www.dataschool.io/simple-guide-to-confusion-matrix-terminology/)

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/)