Documentation: describe how to sign binaries with gpg key
Documentation: describe how to sign binaries with gpg key

In the release document, add a missing section about how to sign
binaries with the CoreOS gpg key.
It's mainly taken from the etcd release document.

coreos/fleet,dongsupark/fleet,jonboulle/fleet,giantswarm/fleet,giantswarm/fleet,coreos/fleet,dongsupark/fleet,coreos/fleet,jonboulle/fleet,endocode/fleet,jonboulle/fleet,endocode/fleet,giantswarm/fleet,dongsupark/fleet,endocode/fleet