Revert removal of Octocat and write a description that might make everyone happy

GitHub :heart:s people that want to use the octocat. The trick is, the Octocat is a trademarked image so to be :sparkles: the use has to be logically inferred as Octocat == GitHub. The copy [original blog post about this book](http://drtortoise.github.io/blog/2012/10/30/c-is-for-computing/) was perfectly aligned with GitHub's [fair use policy](https://github.com/logos).

![](http://distilleryimage5.instagram.com/d0f0dc92229b11e2959322000a9e0b2a_7.jpg)

I've tried to add a description that fits the format being established in this document (which the original copy doesn't really fit into) that and retains the original sentiment of the copy, without sounding too terribly preachy about GitHub (i.e. removing the "cool kids" hierarchical verbiage and promoting "working together" flat verbiage instead).