Document using protobuf from the client

## Expected Behavior

There is no documentation about constructing protobuf messages to issue HTTP requests from the client only HTTPResponses are covered.

## Current Behavior

The documentation and example only cover server side and doesn't cover client.

## Possible Solution

Add more documentation and maybe example app.

## Steps to Reproduce (for bugs)

N/A

## Context

I am trying to implement an app which acts both as http client and server and therefore need to be able to construct protobuf messages and include them in both HTTResponse and HTTRequest.

## Your Environment

* Rust Version (I.e, output of `rustc -V`): 1.43.1
* Actix-* crate(s) Version:
* actix: 0.9.0
* actix-protobuf: 0.5.1

Review the existing actix-protobuf documentation and server-side HTTPResponse example first, then identify how client-side HTTP requests are currently represented. Document constructing protobuf messages for client requests and add a client example if appropriate; done means client request usage is covered alongside the existing response documentation.

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