doc: add a note about the GPG keyserver being unreachable
doc: add a note about the GPG keyserver being unreachable

ehartsuyker/securedrop,ageis/securedrop,micahflee/securedrop,ageis/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ageis/securedrop,garrettr/securedrop,garrettr/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ageis/securedrop,micahflee/securedrop,micahflee/securedrop,micahflee/securedrop,conorsch/securedrop,garrettr/securedrop,garrettr/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop