v0.3.3 Fix docs part 2

Minor README edits to finish bringing the docs into agreement with recent code changes.

v0.3.2 Fix docs

This updates the README docs to match code.py.
Originally, it read from track1.txt, but I then changed it to use track2.txt.

v0.3.1 Fix BPM Math

This fixes the playback timing BPM calculations to be tolerably close to accurate by using scaled integer math instead of software floating point.

v0.3.0 Loop Mode

Changes:

1. Previously, rests at the end of a sequence were not included in the playback time, so looping sounded wrong. Now, rests at the end get included in the playback, so looping works.

2. New switchable looping playback mode: pin board.A0 configured for digital input enables looping playback when connected to GND with a switch or jumper

3. Documentation and demo track updates

v0.2.1 Percussion demo

This adds an mp3 demo of track1.txt being played over USB MIDI into GarageBand, along with a link to the demo from the main README.

v0.2.0 Better Percussion

Main changes since last release:

1. Implement notation for rests
2. Add note name aliases to simplify writing drum parts
3. Assign bass and treble clef defaults to voices (bass: 1, 3, 4; treble: 2)
4. Change the demo track to a moderately complex drum thing
5. Write more README docs about the drum stuff
6. Reduce the amount of debug printing

Bug fixes:

1. Fix MIDI channel assignment off by one error

v0.1.0 initial beta

This works well enough that I expect other people may be able to run it without difficulty. So far, I have only tested with a Trinket M0 (SAMD21) playing USB MIDI to GarageBand on macOS, but I'm hoping it will work just as well in other configurations.

Things that work:

1. Plaintext music notation parsing (creates an array of MIDI events)
2. MIDI event playback with reasonably low-latency timing
3. Debug messages to measure free memory, CPU use, and playback latency

Things that don't work:

1. The music notation parser does not implement rests.
2. Currently, all voices use treble clef ("C" is MIDI note 60). It would be nice to have two voices use bass clef.