Clarify why contributors are listed as authors

Also adding @mcantoni to the list of authors. Sorry we missed you!

Dear contributors,

Even though we weren't able to use your code, we absolutely appreciate
that you wrote it. That's why we're listing you as authors. Thanks!!!

https://dev.metasploit.com/redmine/issues/6034
https://dev.metasploit.com/redmine/issues/5217
https://dev.metasploit.com/redmine/issues/6864