RF: warn about permissions of archive

Suggest running `reown_mpkg` on the built archive. Closes gh-3 - thanks
to @gaborcsardi for pointing out the problem.
Merge pull request #6 from matthew-brett/warn-about-permissions

RF: warn about permissions of archive

Suggest running reown_mpkg on the built archive. Closes gh-3 - thanks to
@gaborcsardi for pointing out the problem.