README.md: C4.1 as process.

I propose to use [C4.1](http://rfc.zeromq.org/spec:22) for this project. Its contents and reasoning are explained more precisely in the [ZeroMQ manual](http://zguide.zeromq.org/page:all#The-ZeroMQ-Process-C).

Codifying the terms of contributing to the project is a Good Thing imho and C4.1 is a battle-proven sensible concept.