Swag: Support readOnly attribute

[wingsofovnia]

It would be great to have readOnly: true supported as a model attribute:

type Car struct {
    ID string `json:"id" readOnly:"true"` // <----
    Name string `json:"name"`
}

[ubogdan]

Looks like "readOnly" attribute it's OpenAPI 2.0 compliant.
I will like to do PR for it but I'm quite busy in this period .
If you really need this feature and you have some time please contribute to swag by writing a PR.

[lvandenbrink]

I would like to have the same feature. I created a pr and added the read only to one of the tests. Let me know if it suffice.

[ubogdan]

Your PR was approved . Thanks for contributing.