cmd/guru: update Emacs installation documentation
cmd/guru: update Emacs installation documentation

I was going to add a hook to enable go-guru-hl-identifier-mode
as a side effect of go-mode, but a person I consulted said that
was bad form; instead I merely documented it.

Change-Id: I48b00b71c7ba485b3d632211923509a74281acbe
Reviewed-on: https://go-review.googlesource.com/24834
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

emacsmirror/go-mode,dominikh/go-mode.el