[release-branch.go1.12] doc: add 1.12 to the project history

Go 1.12 is released, but it's currently not listed in the
https://golang.org/project page.

Change-Id: Ib5820f74245e4c986014c64eb40fa2911473e64b
Reviewed-on: https://go-review.googlesource.com/c/163837


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 467456b0)
Reviewed-on: https://go-review.googlesource.com/c/163727


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>