Auto merge of #4633 - kivikakk:readme-file, r=carols10cents

transmit: send README filename as well as content

This is required to solve https://github.com/rust-lang/crates.io/issues/995; we currently only send the README content, but not the name of the README itself, so it's not possible to determine how we should render it.

I've confirmed the existing crates.io server silently ignores the new field, so this should be safe to roll out whenever.