Document and provide examples how to do multipart/formdata uploads on the client side

**Issue by [jrudolph](https://github.com/jrudolph)**
_Friday Jun 05, 2015 at 16:17 GMT_
_Originally opened as https://github.com/akka/akka/issues/17665_

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Low-level and with marshalling.

/cc @sirthias

Review the existing client-side multipart/form-data support and the low-level and marshalling APIs. Document how uploads are performed and add client-side examples covering both approaches; the documentation should make the complete upload workflow clear.

Written by the indexing model from the issue text.