Add documentation for issue reporters

[nojaf]

We should have a section on how we want users to report problems or suggest issues.
There are issue templates in place but sometimes things we do or don't want are missing some context.
In no order:

• Bugs should be reported using our online tool. Both soundness bugs and `stylistics bugs. If there is a gap between Fantomas and the F# style guide, it should also be reported using the online tool.
• The online tool should not be used for feature requests.
• https://github.com/fsprojects/fantomas/blob/master/CONTRIBUTING.md#what-are-we-not-looking-for should be repeated and expended. Maybe split up and shuffle things around.
• We should also repeat that this is a community project, if you report an issue, you will be asked to tackle it yourself. Again, this needs some context using a friendly voice.