Warn MYMETA in MANIFEST more explicitly

[miyagawa]

#7 and #8 show that many module authors do not know much about MYMETA (since it's relatively new, and isn't documented very well from POV of the module authors), and often accidentally treat them the same way with META files.

shipit could detect MYMETA in MANIFEST and gives warnings in such cases, pointing to more detailed documentation about what MYMETA is, once we get that up somewhere possibly on CPAN module's POD.

Short summary: MYMETA is a end-user version of META.yml/json, which is generated by running configure script (Makefile.PL or Build.PL) to dump the dependencies CPAN clients need to install, etc. It usually includes the same data as META.yml, merged with dynamic requirements, depending on the end-user's local environment.

They must not be checked into a version control repository, MANIFEST nor included in the distribution tarball.

[miyagawa]

Draft: https://gist.github.com/miyagawa/4966350