Add README to /octicons directory with important notes

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.

Note that I've just stubbed out the important content, and more could/should maybe be added to this.
> This directory contains several versions of Octicons for different purposes.

This doesn't feel like it really accomplishes what I believe you are going for, which is explaining the difference between `octicons-local.ttf` and `octicons.tff`. It might be good to just call it out directly:

*If you intend to install Octicons locally, install `octicons-local.ttf`. It should appear as “github-octicons” in your font list. It is specially designed not to conflict with GitHub's web fonts.*

> If you want to use Octicons for your own project, it's best to use the generator at [octicons.github.com](http://octicons.github.com/).

I don't feel that this is correct. This *is* the best place for putting octicons in your own project, if you want to use the bower package. If you aren't here for the bower package, then octicons.github.com is the best place. TBH, this part could be left out, I think.