Add note about AppArmor restricting unprivileged namespaces in Ubuntu 24.04 to README

Commit 18a365c addressed this in CI, but it took a couple minutes to find the (arguably insecure) fix myself, so it would be convenient to mention this in the docs. Here’s the original bug report I wrote on this:

Starting in Ubuntu 23.10, [AppArmor](https://discourse.ubuntu.com/t/spec-unprivileged-user-namespace-restrictions-via-apparmor-in-ubuntu-23-10/37626) prevents unprivileged applications without a profile from creating namespaces.

This causes `./configure` to fail with “try needs unshare to work, but could not run unshare.”

The easy solution is to run `sudo sysctl kernel.apparmor_restrict_unprivileged_userns=0` to disable the apparmor restriction. Another solution would be to provide a PPA or a .deb installable that sets up the apparmor profile.