[release-branch.r60] doc: release.r60

««« CL 4981047 / c0fea223bc90
doc: release.r60

R=dsymonds, r, rsc
CC=golang-dev
https://golang.org/cl/4981047
»»»

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4961072