Add README note on Ubuntu's restricted unprivileged user namespaces

As encountered on the latest LTS (24.04); see https://ubuntu.com/blog/ubuntu-23-10-restricted-unprivileged-user-namespaces

Can be disabled using sysctl:

```
kernel.apparmor_restrict_unprivileged_unconfined=0
kernel.apparmor_restrict_unprivileged_userns=0
```

I haven't considered the alternative, e.g., using an AppArmor profile.