Create a guide for installing & creating WP CLI packages

[roborourke]

Originally raised by @ntwb in humanmade/altis-local-server#118

We should provide some guidance on installing WP CLI command packages and the basics of how to create your own.

Should include:

• What is WP CLI
• Some common command examples (adding a user, search replace, listing post ids and passing to another command)
• Use cases for custom commands
• Adding command packages via composer instead of wp package install & why (not supported on production infrastructure)
• Using wp package list to find packages, or https://packagist.org/?type=wp-cli-package
• Common problems eg. if the command doesn't check for existence of WP_CLI class
• How to create your own command