Add images and modify readme accordingly

see the rendered version: 

https://github.com/ptone/bc/tree/illustrated-readme

In addition to the images - I fleshed out the contribute back part, and tweaked a few words.

desktop -> local
called -> named

see diff for full changes

Rather than merge these changes into master/readme.md as well, I'd modify the master branch readme to be just a short note telling people that all the action is on the gh-pages branch, and to git checkout to that.

Now that gh-pages is the default branch on GH - you don't have to worry about the master version of the readme as being something people will easily stumble upon.