Doc: User namespace are disable by default in debian

[ikus060]

We should leave a notice in the README about Debian hardening regarding userns.

To enable, we need to:

echo 1 > /proc/sys/kernel/unprivileged_userns_clone

and make it persistant using:

echo 'kernel.unprivileged_userns_clone=1' > /etc/sysctl.d/userns.conf

I'm also reading a similar path is available for newer kernel with a different switch.

[radhermit]

We should leave a notice in the README about Debian hardening regarding userns.

I don't use Debian so it would be helpful if you make a PR adding a subsection to the requirements section of the README.