Support for YARD documentation

This patch:

  - Adds `yard` to the development group of the Gemfile. `redcarpet` is added
    as well in order to support Markdown formatting.

  - A `.gitignore` file is started which includes YARD and Bundler artifacts.

  - A `.yardopts` file is started which lists non-code documentation.
Start a Getting Started guide

This guide will walk through transitioning a small Vagrantfile to
vagrant-config_builder and provides a jumping off point for the discussions of
things such as formats, models and filters.
docs: Expressing Configuration as Data

This section gives an example of converting a Vagrantfile to ConfigBuilder
YAML. It also discusses how loaders work.
Merge pull request #18 from adrienthebo/moar-docs

Start YARD documentation