add Pod encoding marker

Hi,

The lack of explicit encoding marker causes mojibakes in place of non-ASCII symbols on [sco](http://search.cpan.org/~jfried/Rex-0.43.7/lib/Rex.pm): `Boris DÃ¤ppen`. [MetaCPAN](https://metacpan.org/pod/Rex) gets it right, but warns at the bottom of the page.

Explicit encoding marker should fix both of those.

Cheers,
Sergey