Plugins are centric to the RubyServ ecosystem because it is what handles how your services will function.
Plugins go into the plugins directory, and the best example of one of the plugins is RubyServ itself, found in plugins/core.rb.
The only thing required in a plugin are include RubyServ::Plugin and configure. The module name doesn't matter, but stay out of the RubyServ namespace to avoid namespace clashes.