recommend setting GOROOT in golang installation

I found it necessary to override the default GOROOT to be in the libexec (source) directory rather than letting it be in the homebrew go root dir.

An example:
```shell
 ~/dev⮀ export GOROOT=/usr/local/Cellar/go/1.2/
 ~/dev⮀ brew install camlistore
==> Downloading https://github.com/bradfitz/camlistore/archive/0.6.tar.gz
Already downloaded: /Library/Caches/Homebrew/camlistore-0.6.tar.gz
==> go run make.go
	/usr/local/Cellar/go/1.2/src/pkg/strings (from $GOROOT)
	/Users/ranman/gopath/src/strings (from $GOPATH)
make.go:45:2: cannot find package "time" in any of:
	/usr/local/Cellar/go/1.2/src/pkg/time (from $GOROOT)
	/Users/ranman/gopath/src/time (from $GOPATH)
```

vs.

```shell
~/dev⮀ export GOROOT=/usr/local/Cellar/go/1.2/libexec
~/dev⮀ brew install camlistore
==> Downloading https://github.com/bradfitz/camlistore/archive/0.6.tar.gz
Already downloaded: /Library/Caches/Homebrew/camlistore-0.6.tar.gz
==> go run make.go
/usr/local/Cellar/camlistore/0.6: 11 files, 80M, built in 8 seconds
```