add ContributorsFromGit dzil plugin to dist.ini

This won't add a section to the POD, as we're not using Pod::Weaver, but this
will populate the x_contributors metadata.  metacpan picks up on this and
displays it :)
Merge pull request #3 from RsrchBoy/topic/contributors-from-git

add ContributorsFromGit dzil plugin to dist.ini