Add guidelines about content negotiation

[MassimoC]

provide guidelines on content negotiation (accept / content-type).

[MassimoC]

Introduce ConsumesAttribute and ProducesAttribute

Having the operations decorated with these attributes should also affect the Swagger documentation.

For example:

[SwaggerResponse((int)HttpStatusCode.OK, "Player data object")]
[SwaggerResponse((int)HttpStatusCode.NotFound, "Player not found")]	        
[SwaggerResponse((int)HttpStatusCode.InternalServerError, "API is not available")]
[Produces("application/json")]
public async Task<IActionResult> GetPlayer(int id) {}

What's the point to have application/json and text/json?
We should remove text/json

[fgheysels]

Regarding text/json, application/json: I just took that over from the code at hand. Also to show how multiple contenttypes can be specified.
But for me, it is OK to drop text/json.

[MassimoC]

Ref #102 and #103

[tomkerkhove]

@FilipVanRaemdonck is working on this. Pending invite to assign to him.