Update "contributing" and "development" documentation

# Overview

This change was motivated to supply an answer to #26.

While #26 is ostensibly about a test suite not passing, I think the bigger issue is that there's no clear description for how to get started with local development.

And so-- this change contains an update to doc/DEVELOPMENT.md and the creation of doc/CONTRIBUTING.md and the outright linkage of README.md to doc/CONTRIBUTING.md for future developers.
Add a `## Design` section that talks about making use of common ruby libraries, emoji, htmlpipeline, filters, and tilt. Create a `### Markdown` section describing how markdown. It is the most popular slide format, and it's the most heavily customized.