godoc/dl: add instructions for unstable "go get"
godoc/dl: add instructions for unstable "go get"

Adds instructions for "go get golang.org/x/build/version/..."
for the unstable version.

Fixes golang/go#18337.

Change-Id: Iea5e8fab7f00a053e3b42ef355277307addb635b
Reviewed-on: https://go-review.googlesource.com/34823
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools