Add an example of a free guest subscriber.
user
```ruby
require 'noam-lemma'

# This is an example of a Ruby Lemma that publishes message and *also* uses the
# "Guest" model of connection. This Lemma will advertise that it's available on
# the local network and only begin subscribing to messages once a server
# requests a connection from the Lemma.

publisher = Noam::Lemma.new(
  'example-publisher',
  'ruby-script',
  9000,
  ["e3"],
  [])

# Using the `advertise` method asks the Lemma to announce it's presence and
# wait for a message from a server that may want to connect to it.
#
# The "local-test" parameter is the room name. Servers with a room name that's
# the same as the Lemma's advertised room name will connect automatically.
publisher.advertise("local-test")

loop do
  # The `listen` method will return an Event object once one is received by the
  # Lemma. Until an event is heard, the `listen` method blocks.
  m = subscriber.listen

  # There's one special value that's returned from `listen`: the `:cancelled`
  # symbol. If this shows up, it means some one else has called the `stop`
  # method on the Lemma.
  if :cancelled == m
    puts "Done"
    break
  else
    puts "Read: #{m.ident} -> #{m.value.inspect}"
  end
end
```
assistant