Changed link

Mathematics for Computer Science (November 2013 Version) old link directed to a page that requested password, used a new link that doesn't require one.
Merge pull request #1124 from JIGD/master

Changed link
Added Raspberry Pi User Guide

This User Guide is written by Eben Upton, Co-Creator of the Raspberry Pi
Merge pull request #1126 from kumaresa18/master

Added Raspberry Pi User Guide