update_demoweb_howto.txt

To make updates to the demoweb DB (for example if the SQL structure changed):

- run update_demoweb.sh to ensure the DB state is fresh
- make the necessary changes
- run dump_demoweb_db.sh to create a new file ~/demoweb/domjudge_demo-YYYY-MM-DD.sql
- check that the new file differs from the previous dump in the expected way
- update the symlink ~/demoweb/domjudge_demo.sql to point to the new dump
- run update_demoweb.sh to test that the new file is loaded correctly