Document advantages over cloud-init or alternatives

[Lucas Ramage]

What advantages come from using yip over cloud-init directly? Are there any other alternatives to consider? e.g., ignition/butane

[Ettore Di Giacinto]

Thanks @oxr463 ! that's a good point that is totally missing in the docs

[Olaf Meeuwissen]

Tangentially related, document what subset of cloud-init that is implemented.