[release-branch.go1.5] doc: add Go Security Policy document

Bring in the text from the proposal (with minor edits):
https://github.com/golang/proposal/blob/master/design/11502-securitypolicy.md

Fixes #11502

Change-Id: I92a987be66a0df60c1fad6c6c79f89bd8e9c12a8
Reviewed-on: https://go-review.googlesource.com/13955


Reviewed-by: Jason Buberel <jbuberel@google.com>
Reviewed-on: https://go-review.googlesource.com/14224


Reviewed-by: Andrew Gerrand <adg@golang.org>