add README to directory

This README can contain important clarification about how to use this directory for users who might land here not realizing that a) it contains versions of Octicons for different purposes (web vs local), and b) http://octicons.github.com exists as the best place to download the fonts.
reword for better clarification
Merge pull request #12 from jglovier/patch-1

Add README to /octicons directory with important notes