[release-branch.r57] doc: release.r57

««« CL 4457049 / d222384d1bad
doc: release.r57

R=golang-dev, r, adg, r2
CC=golang-dev
https://golang.org/cl/4457049
»»»

R=golang-dev
TBR=r
CC=golang-dev
https://golang.org/cl/4431087