Add a CPAN-project friendly .gitignore and MANIFEST.SKIP

I usually forget to do this on my own projects, so here's
a .gitignore and corresponding MANIFEST.SKIP that might
save you some grief in the future.

Background info: if you accidentally release a project to
CPAN that contains MYMETA.{json,yml} then other people can
run into problems installing your module. You haven't done
that, which is great! This commit will ensure nobody accidentally
submits a pull request that does that.