[release-branch.go1.9] doc: document Go 1.9.6
    
    Change-Id: I9699b22d3a308cda685aa684b32dcde99333df46
    Reviewed-on: https://go-review.googlesource.com/110315
    Reviewed-by: Filippo Valsorda <filippo@golang.org>
    (cherry picked from commit 40685b230a012dd5a9ca471dbf21b65457c32147)
    Reviewed-on: https://go-review.googlesource.com/110375
    Reviewed-by: Andrew Bonventre <andybons@golang.org>