Clarify behavior of chords with notes of different durations

[mdgood]

MusicXML 3.0 allows chords with notes of different durations, with the longest duration appearing in the first note before any notes follow using the <chord/> element. This behavior could be clarified in two ways:

• Clarify that the next main note follows after the main note's duration, not a shorter note duration in a chord note. This could be part of a general clarification that chord notes do not move the MusicXML part's timeline.
• Motivate the need for this feature with repertoire such as multiple stops for stringed instruments (e.g. Mozart's Concerto No. 3, K. 216).

See the discussion started by Terence Kelly on the MusicXML forum at http://forums.makemusic.com/viewtopic.php?f=12&t=1922.

[mdgood]

Pull request #402 should fix this issue. I will likely merge it soon, then reopen this issue so that people can review it more easily in the new documentation and suggest any improvements here. The documentation was updated for the chord and duration elements, and a new example added which shows different note durations within a chord.

[mdgood]

Reopening for review comments now that the pull request is merged.