Sample/Doc for pipeline module(s)

[samsp-msft]

Most of the questions we get for "how do I?" with the proxy can be solved with a custom middleware component. Although we have a samples folder - that is more of a scratch pad for the teams internal experiments - it is not documentation for what you can do and how you should do it as a middleware component. For example, can you create new destinations on the fly, can you lookup the destinations for a backend cluster and swap those in.

This should cover:

• What properties existing pipeline stages operate on, and write out -> so that you know where to insert your module
• Reading from config
• Modifying destinations for a request
• Reading and modifying headers before the request is sent to destination
• Reading and modifying headers before the response is sent to the client

[karelz]

Duplicate of #114