Create a guide for installing & creating WP CLI packages

Originally raised by @ntwb in https://github.com/humanmade/altis-local-server/issues/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