Add a "notes" field to YAML format

[ataseren]

I think there should be a field for additional notes about YAMLs, especially complex ones. Some tools asks for optional steps such as providing API keys and custom options. Instead of adding parameters to a playbook, we can add detailed additional notes of how to perform these optional steps.

Also, I think we shouldn't use "description" field for this purpose since the content under this field is displayed on UI and it may mess up the UX in case of long notes.