Add usage documentation for pitch argument

lib/music_theory/note.rb

@@ -9,7 +9,7 @@ class Note
 
  def initialize(options = {})
  @frequency = options[:frequency] || 440.0 # Note frequency in Hz
- @pitch = options[:pitch]
+ @pitch = options[:pitch] # ex: Pitch.new(:D, 7)
  @frequency = pitch_object_frequency
  @duration = options[:duration] || 1.0 # Number of seconds per note
  @distort = options[:distort] || false