Update README.rst

Added missing imports in example README.rst

Author: sepro

README.rst

@@ -61,6 +61,11 @@ Usage Example
     import usb_midi
     import adafruit_midi
 
+    from adafruit_midi.note_on import NoteOn
+    from adafruit_midi.note_off import NoteOff
+    from adafruit_midi.pitch_bend import PitchBend
+    from adafruit_midi.control_change import ControlChange
+
     midi = adafruit_midi.MIDI(midi_out=usb_midi.ports[1], out_channel=0)
 
     print("Midi test")