/usr/share/doc/isc-dhcp-client-dbg/README.Debian is in isc-dhcp-client-dbg 4.1.ESV-R4-0ubuntu5.10.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/isc-dhcp-client-dbg/../isc-dhcp-common/README.Debian