Workshop exercise on domain modelling.

• cd to repo's root directory and use irb to require './notebook.rb'
• you can create a notebook by notebook = Notebook.new
• you can then interact with it in 3 ways:
  • notebook.create_note to create a note. when saving, you're prompted to save it with a tag, and also reminded of all existing tags.
  • notebook.search_notes returns all your notes, one after the other
    • if you want to filter your search, you can call this with a string argument, which is the tag you made when you created a note.
    • notebook.search_notes("sports")
  • notebook.print_note_count to show you how many notes you have.