Cryptocat guide materials

Here are some graphics files that can to be used in Cryptocat guide materials (both print and web).  The main thing for these are really just the diagrams. I included both the image files for different facets of the graphics, image files of the completed diagrams, and PDFs of how these work in page spreads in a quarter-sheet sized pamphlet that I'm working on.

![how_cryptocat_works](https://f.cloud.github.com/assets/1476785/999061/2798ab20-0a1c-11e3-89e0-a6300cb759d6.png)

Also edited main readme for the meta repository and added a readme for the guide documents.