This change makes the release notes for Go 1.13 more complete
by mentioning a new function in the os package.

Change-Id: I0d637fd70ff6d14782bbfb7c13985a0f83b19d6d
Reviewed-on: https://go-review.googlesource.com/c/go/+/181945Reviewed-by: Ian Lance Taylor <iant@golang.org>