contrib: improve motd for nm-in-container.sh script
Leave a hint about core-dumps. Also, now we have `contrib/fedora/rpm/configure-for-system.sh` script, which can configure the build in a way similar to what we get when doing an RPM build. That means, inside "contrib/scripts/nm-in-container.sh" we can just type `make install`, and it will replace the pre-installed NetworkManager. The main advantage is that it becomes convenient to run NetworkManager as a systemd service. Previously, the suggested was to to install NetworkManager inside another prefix, and run it in the terminal. Running NetworkManager as systemd service is also necessary for NM-ci, which restarts the NetworkManager service, and you couldn't run a test, if you just started NetworkManager in a terminal. Previously, you had to build a complete RPM, which takes a lot of time.