/usr/share/doc/mkvtoolnix/README.source is in mkvtoolnix 6.7.0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/mkvtoolnix/../quilt/README.source
/usr/share/doc/mkvtoolnix/../quilt/README.source