Add about page for custom metadata MetaCPAN supports

This adds an about page listing the custom metadata fields that MetaCPAN supports.  Currently, this includes x_contributors and x_IRC.  I've also listed information about the handling of change logs, linking to the currently nonexistent CPAN::Changelog module.

I'd be interested in feedback on the current text or for things to add to this document.
s/an link/a link/