encoding/gob: note pacakge not covered by security policy

encoding/gob: note pacakge not covered by security policy

And add a link. Resolves a comment left on http://go.dev/cl/436096
after it was submitted.

Change-Id: I2847d29134ffb4fee2b0ea37842cdf57df55ec0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/442816
Reviewed-by: Julie Qiu <julieqiu@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>