Improve spray migration guide on marshallers

Feedback via twitter: https://twitter.com/oxbow_lakes/status/798106140483735552?s=03
Writeup here: https://gist.github.com/oxbowlakes/117f0dbfe8eb05347fd25db9ca3c4cd3

It's about that we should explain the marshallers in more depth, imports and context.
Help on improving the guide would be very welcome!

Start with the Twitter feedback and linked Gist, then find the spray migration guide in the documentation. Expand the guide with deeper explanations of marshallers, imports, and context; done when those topics are clearly covered for readers.

Written by the indexing model from the issue text.