Improve ModuleResults documetation

[bekh6ex]

Currently, there are 5 different pieces of data that a module provides after processing: severity_level, code, offender, description, mitigation.
In some cases, it is not clear what data should be in each one of these fields, for example:

• What is the purpose of code and offender?
• How long the text in description should be? What information should it contain?

If class ModuleResults had documentation explaining the purpose of each field and explaining the difference in severity levels it would be much easier to write a new module. It becomes even more important in the context of #35, if modules' code might be located in another repository.
And also, it would improve the end-user experience in terms of consistency of results.

As a Hawkeye module developer
I want to quickly understand what data should my module return
So that I don't need to go through the code of other modules
And also reduce pull request review time (reduce 'pr-requires-changes' possibility)
And also make Hawkeye output more consistent across different modules