Finalizing metadata

Warnings were handled on this branch as well. In this case I didn't bother resetting the score layout since with the amount of complex cross-staff writing it would have looked horrid. See what you think.

enriches readme with historical background and harmonic observations

I appreciated the note about this repo as a test of the 'extreme' of the annotation standard and I see how it's provided a useful basis for our 20th-century efforts since then!

Update corpus with ms3 2.4.0

That involves adding the chords facet for accessing, not chord labels, but articulation, dynamics, etc.