[dput-ng-maint] Bug#863760: dput-ng: document return codes

[Nico Schlömer]

Package: dput-ng
Severity: minor

dput-ng often fails to upload with output like
```
dput -c /path/to/dput.cf mypackage mypackage_1.0.0-1xenial1_source.changes
Return code:
3
Output:
None
```
The return code may differ, and it's always unclear what it actually means.

Please document dput-ng's return codes.



-- System Information:
Debian Release: stretch/sid
  APT prefers zesty-updates
  APT policy: (500, 'zesty-updates'), (500, 'zesty-security'), (500, 'zesty'), (100, 'zesty-backports')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.10.0-21-generic (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)