Add OutputFormatter

[JohnGalt1717]

It would be really great if this included an Output formatter or if there was sample code on how to create an output formatter for asp.net core.

[dodyg]

@JohnGalt1717 https://github.com/dodyg/practical-aspnetcore/tree/master/projects/mvc/output-formatter-syndication

This is a very rudimentary RSS output formatter. The problem is that SyndicationFeedReaderWriter does not provide "frame" for the syndication contents to attach to which will be necessary for a proper Output Formatter to work.

Right now there is no built in Syndication that you can just return Ok(syndication) at the Controller.