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. (*The problem seems to replicate across any non-default shell*). 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.
We try to keep the Homebrew caveats limited to stuff that's Homebrew specific i.e. only applies to the Homebrew `go` formula and not other Go distributions.