Improve the README
Improve the README

Add new variables and their description.
Explain how to add services and filters in `group_vars` and `fail2ban_filters` respectively.

buluma/trellis,newtonne/trellis,roots/trellis,newtonne/trellis,roots/trellis,buluma/trellis,pressbooks/trellis,roots/bedrock-ansible,roots/bedrock-ansible,newtonne/trellis,buluma/trellis,pressbooks/trellis,buluma/trellis