/usr/lib/x86_64-linux-gnu/libreadline.so is in libreadline6-dev 6.3-8ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /lib/x86_64-linux-gnu/libreadline.so.6
/lib/x86_64-linux-gnu/libreadline.so.6