Adding skull_island CLI tool

[jgnagy]

Adding a link to the Skull Island CLI tool and Ruby SDK. The tool is awesome because:

• I'm biased (I wrote it)
• It is both a CLI tool and an SDK for the Kong Admin API
• It can help people migrate to 1.1+
• It supports YAML + embedded ruby (it allows for limited templating)
  • Specifically, IDs can be looked up using lookup(), allowing portable configuration
• The YAML configuration supports a project key that can be used to add, update, or remove configuration
  • This leverages special, hidden tags under-the-hood, meaning maintaining state is done within Kong itself
  • This works wonderfully from a CI perspective because it is easy to have a build stage that triggers skull_island import on project-specific Kong config
• It is open-source!