[release-branch.go1.9] doc: document Go 1.9.7

Change-Id: I10dededb41162a99dbc42792a9d314c6dabddcca
Reviewed-on: https://go-review.googlesource.com/116362


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
(cherry picked from commit b3a9948126471ac285e30c7920465c4c28ea0526)
Reviewed-on: https://go-review.googlesource.com/116475


Reviewed-by: Filippo Valsorda <filippo@golang.org>