Hi,

This adds emacs-lilypond-mode. The mode is distributed as part of lilypond,
so I'm using the same pattern that emacs-protobuf-mode and
emacs-erlang-mode uses. Reuse the parent package, changing only the
build-system and adding the emacs files to the load-path.

Additionally I've verified that I can use the package by including the
following in my emacs' init.el

(use-package lilypond-mode
  :mode ("\\.ly\\'" . LilyPond-mode))

Cheers

-- 
"I object to doing things that computers can do." — Olin Shivers