add information comparing the compiler implementations to the installation document
add information comparing the compiler implementations to the installation document

R=rsc
CC=go-dev
http://go/go-review/1018063

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go