README should focus on proxy toolkit rather than the example proxy implementation

[admtnnr]

Martian is, first and foremost, a set of tools to build custom HTTP/S proxies.

We've always had an example proxy implementation that we've maintained to show others how one could be built which has been confusing to users. We should change the README to present Martian as a toolkit for custom proxies rather than the documentation for the example implementation.

At some point we may want to move the testing proxy implementation to a separate repo and name it something else.