Note which fields can be used with environment markers

[takluyver]

PEP 345 lists six fields which can have environment markers. I've noted it for all six, but I don't know if I should. In particular, I've never seen a classifier listed with an environment marker, and it doesn't make much sense to me to allow that.

I've also updated the specification of Requires-Dist with a brief summary of PEP 508.

Closes gh-409

[theacodes]

Thanks @takluyver.

Re:

In particular, I've never seen a classifier listed with an environment marker, and it doesn't make much sense to me to allow that.

Perhaps @ncoghlan or @dstufft has some insight here.

I'm going to go ahead and merge- we can address post-hoc comments in a follow-up PR.