doc: refer to wiki page for downloads
doc: refer to wiki page for downloads

We will serve downloads from here until we work out a better plan.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/95980044

sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh