[WireGuard] README.md refers to doc/protocol.md

[Jason A. Donenfeld]

Hey dkg,

Thanks for finding that. Indeed I initially had all the docs in docs/
and moved it to wireguard.io for the public release. They still live
in markdown format and the site is built from that. Right now I have
no plans of moving the documentation back into the repo. In any case,
I've got a good system for keeping the docs in wireguard.io up to date
with the wireguard src repository; they're both versioned in git, and
perhaps I'll open up the repo for the site markdown documentation.

Good feedback, thanks.

Jason