Add minimal set of documentation

[scottslowe]

In order to make it easier for other people to contribute to Wardroom, we need to add documentation to Wardroom. This documentation, at least in the first iteration, should provide the following information:

• An overview of Wardroom's architecture
• Instructions for building images using Wardroom (can pull from existing content in the packer directory)
• Instructions for cluster bootstrapping (this needs to be written, AFAICT)
• Instructions for performing tests against Wardroom (will be a mix of content from the existing main README.md plus new content as additional tests are added)

[jpweber]

Here are a few things I've hit that I had to figure out that would have been nice if it was documented

• how to specify your podCidr for the kubeadm config
• how to change your podCidr in for the calico config
• fields that a user is likely to need to modify / add to the extra_vars.yaml file. such as kubernetes_common_api_ip _api_fqdn _primary_interface. Also version for things