[Ottoman] - Add section on how to generate a UUID for a field

https://github.com/couchbaselabs/node-ottoman/issues/640

Questions around "how to generate UUID for a field " pop up quite often.
It's difficult to infer this information from the current documentation.

We should add a section in the documentation which is searchable and at the same time has example around it.

As an example for a field _id to use UUID it's as simple as attributing it as auto : 'uuid'

 _id: { type: String, required: true, auto: ‘uuid’ }

Maybe we could add this in the Ottoman Getting Started page? This is for the ottoman documentation, not our docs site.