Document the file URLs options in the configuration

[odupuy]

The help shows URLs like

"apis": [
    {
      "url": "http://petstore.swagger.io/v2/swagger.json"
    },

On Windows, you can use a relative URL e.g. "url": "file://tmp/src/swagger-myapp.yaml" or an absolute one e.g. "url": "file:///c/Users/olivier/go/src/github.com/myrepo/myproject/tmp/src/swagger-myapp.yaml"
On Linux, you can use only an absolute one!
Thanks