Documenting Instruments.xml

The instrument list is starting to look good! With the work of ChurchOrganist to clean the list and add MusicXML ids and then the work of MarkRS to filter instruments by genre, there have been questions about the structure of the instruments.xml list and how each tag are used.

For MuseScore 1.3, we had a small and pragmatic documentation of the instruments.xml file. For 2.0, the format changed and ChurchOrganist started a XML schema description, which is nice for "developers" but not really useful for intermediate users. I would propose to write a description of the full format collaboratively.

I started a shared document. Jump in!