Add more documentation

[selfthinker]

We don't have any documentation which tells people how to make any changes. This adds information in a CONTRIBUTING file on how to

• update existing test results
• change existing code examples
• add new code examples
• add a new tool and the requirements for including it

and what to generally look out for when testing a tool.

And it adds another file which lists changelogs and options to use for each tool.