Add example and more detailed description for GenRMQ.Processor

[vorce]

Description

I think we have not explained the purpose of the Processor behavior clearly. This PR adds an example implementation and at least some more description of the "why" for GenRMQ.Processor.

Checklist

• I have added unit tests to cover my changes.
• I have improved the code quality of this repo. (refactoring, or reduced number of static analyser issues)
• I have updated the documentation accordingly

[coveralls]

Coverage remained the same at 89.113% when pulling aab87e2 on processor-example into a6fe01f on master.

[mkorszun]

Cool, thanks 👍 This module was a little bit forgotten :)