doc additions power chords (issue2686041)

[Carl . D . Sorensen]

Reviewers: ,

Message:
Patrick Schmidt has added two- and three-note power chords to the known
set of chord names.

He's provided both code and docs.

Please review.

Thanks,

Carl


Description:
doc additions power chords
additions to fretted-strings.itely description and examples for the use
of the new command \powerChords
addition of two new modifiers to notation-appendices.itely

power chord symbol
Added power chord definition, symbol and command to typeset this symbol.

Please review this at http://codereview.appspot.com/2686041/

Affected files:
  M Documentation/notation/fretted-strings.itely
  M Documentation/notation/notation-appendices.itely
  M ly/chord-modifiers-init.ly
  M ly/property-init.ly