Document what to do instead, if using the (anti pattern) respondWithMediaType from Spray - Explain the benefits of marshalling

**Issue by [ktoso](https://github.com/ktoso)**
_Thursday Oct 01, 2015 at 12:35 GMT_
_Originally opened as https://github.com/akka/akka/issues/18625_

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Hi @sirthias @jrudolph,
I realised we don't have `respondWithMediaType` ported to Akka HTTP and am expecting that this is because we want people to rely on using the content type negotiation that HTTP does instead of baking their own using this directive...?

I'd like to confirm my thinking about it so I can sleep peacefully at night :wink:

Refs:
- http://spray.io/documentation/1.2.2/spray-routing/respond-with-directives/respondWithMediaType/
- https://github.com/akka/akka/issues/18425

Read the linked Spray respondWithMediaType documentation and issue 18425 first, then inspect the Akka HTTP documentation for content negotiation and marshalling. Done means the docs explain why respondWithMediaType is not provided, what to use instead, and the benefits of marshalling.

Written by the indexing model from the issue text.