Document the diagnostics used for Chad's setup

Chad had an issue: his GPGME expected the `gpg` program to be in
`/usr/local/opt/gnupg/bin/gpg`, but it was instead in
`/usr/local/opt/gnupg2/bin/gpg2`. Unclear how this happened.

We tracked it down like so:

    env GPGME_DEBUG=7 ./src/mkmnfst manifesto.txt