Add documentation and example about enumerate attributes

[sbrunner]

View code https://github.com/camptocamp/c2cgeoportal/blob/master/c2cgeoportal/views/layers.py#L413

Example of query:
.../layers/{layer_name}/values/{field_name}

Example of result:

{
  "items": [{
    "label": "<name>",
    "value": "<value>"
  }, ...]
}