Add documentation on Psalm v6 including similar functionality

[cspray]

Psalm 6 has a check for whether or not a non-public class that has not been extended has been marked final. The check, ClassMustBeFinal, is very similar to what this plugin offers, with a few nuanced differences. More research should be done on this new check available in Psalm v6. Then we should document whether or not this plugin should continue to be used in v6 and, if so, any changes that might be available to improve the experience.