docs: add some fields to the README

[jablko]

Does it make sense to add the manifest _id field and the packument _cached and _contentLength fields to the README, along with the other fields that Pacote adds? That's what this PR does.

[wraithgar]

This is pretty good timing because _id came up in #176.

The npm cli already assumes _id is always set when things come back from pacote, it's probably worth documenting.