Describe the file format of core metadata

[takluyver]

Following this discussion on discourse. The file format of core metadata is not precisely defined (PEP 241 said RFC 822, but that's outdated in practice), but there is a practical standard: the major tools consuming metadata use the email.parser module with the (default) compat32 policy. I think it's better to describe the situation than to leave it completely unspecified.

Hopefully this is an interim measure until someone gets around to describing the format properly. But that's a much bigger job, and so far no-one is volunteering.

[pyfisch]

While I've started a discussion to specify the format in greater detail there are two things that can be checked to ensure invalid files are rejected by tools:

1. Make sure the defects attribute is the empty list.
2. Check if the message contains both a Description header and a non-empty payload. This is a unambiguous sign that the message syntax is invalid.

[webknjaz]

I think this change is good and any additional changes can be submitted as follow-up PRs.