Go Caveats

I don’t particularly love that this isn’t the cleanest set of warnings
in the world, but the combination of Homebrew, zsh & Go seems to really
screw up the setting of the GOPATH & the $PATH as recommended. I’ve
chucked in a few lines consequently about how to handle non-default
shells.

I’ll respect if this somewhat cumbersome explanation is rejected, but
hopefully it’s useful to include.
go: update 1.3 bottle.