Document how to use Protobuf (or something else) as a MediaType

**Issue by [aiyanbo](https://github.com/aiyanbo)**
_Tuesday Dec 16, 2014 at 07:34 GMT_
_Originally opened as https://github.com/akka/akka/issues/16548_

---

We used Protobuf encoding objects on my project. I marked data format as a MediaType, like this:

```
Content-Type: application/x-protobuf
```

Start by locating the documentation for MediaType and the existing content-type examples. Document how users can represent Protobuf, or another custom format, as application/x-protobuf, and make the supported usage clear to readers.

Written by the indexing model from the issue text.