Bug#952657: Document OpenSSL linking exception

[Bastian Germann <bastiangermann@fishpost.de>]

Package: libqt5network5
Severity: normal

This package links to libssl. An OpenSSL linking exception is included
in src/network/doc/src/qtnetwork.qdoc and files in src/network/ssl but
it is not documented in debian/copyright. Please include it there.

Please also have a look at #924937. libqt5network5 has many GPL licensed
reverse dependencies, similar to libpq5, some of which do not have this
exception. Please consider configuring with -openssl-runtime because
that would make this issue less important for these reverse dependencies
Then one could install them without libssl, which would not make them a
derivative work of OpenSSL.