Stack API documentation

@fermayo 
I think this is what might need rethinking:
  - Tags at the stack level (maybe is better to ignore them for now).
  - Stack API gets `json`documents, and each service in the stack POST operation is on the same format as a `Create new service` operation. Therefore, the translation from the `yaml` syntax to `json` needs to be done by the client. I propose this for API consistency and also, the translation is pretty straightforward.