document pgp limitation about only supporting gpg1
document pgp limitation about only supporting gpg1

we currently only support gpg1 and have an unknown host of compatibility
issues with gpg2. advise that users make sure the gpg1 packages are
installed to use aptly.

relates to aptly-dev/aptly/issues/657

aptly-dev/aptly-dev.github.io,aptly-dev/aptly-dev.github.io