[pkg-gnupg-maint] Bug#850982: Add instructions to disable gpg-agent user service in README.Debian

[Yuri D'Elia]

Package: gnupg-agent
Version: 2.1.17-4
Severity: normal

The gpg-agent and dirmngr services are now auto-enabled for user sessions,
which is actually a nice improvement.

Can we tweak the instructions present in the README.Debian to include the
commands required to disable this for a single user, and also globally?

I do not want to auto-start these services for the root user. I also want to
disable auto-start completely in servers I'm logging into. I think both are
pretty common scenarios and deserve special mention, as systemctl --user
disable won't work some might expect.

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (900, 'unstable'), (800, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-trunk-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages gnupg-agent depends on:
ii  libassuan0                2.4.3-2
ii  libc6                     2.24-8
ii  libgcrypt20               1.7.5-2
ii  libgpg-error0             1.26-1
ii  libnpth0                  1.3-1
ii  libreadline7              7.0-1
ii  pinentry-gtk2 [pinentry]  1.0.0-1

Versions of packages gnupg-agent recommends:
ii  gnupg  2.1.17-4

Versions of packages gnupg-agent suggests:
pn  scdaemon  <none>