docs(doc/userguide.md): tweak Debian installation — radicle-ci-broker

Add note about kvm kernel module and Unix group.

Add step to create log directory.

Signed-off-by: Lars Wirzenius liw@liw.fi

   - `curl -sSf https://radicle.xyz/install | sh`
   - `rad auth`
   - `rad node start`
 * Make sure the `kvm-intel` or `kvm-amd` kernel module is loaded, so
   that VM hardware acceleration works. You may need to add your user
   to the `kvm` group.
 * Make sure the node has at least a test repository:
   - `rad seed rad:z28U8KUBvVSMQc13NydX3LBDsdEdQ`
 * Install relevant software from `deb` packages on <http://apt.liw.fi/>:

 base_url: "https://ci0.liw.fi//ambient-log"
 ~~~
 * Create `/srv/http` with write permission for your user:
   `sudo install -d /srv/http -o $USER -g $USER`
 * Start the CI broker in the foreground:
   - `cib --config ci-broker.yaml --log-level info process-events`
 * In another terminal or shell sessions, trigger a run: