Make README less LilyPond-centric

Fixes #1
Update git-workflow.md

Based on previous experiences we actually *want* all development
to happen on master.
Setup instructions: first draft
Setup instructions: continue
setup.md: Add a few clarifications
Misc changes
Rename some files to show their order of reading
Move stuff around
More reorganization and renaming.  this is very messy
Put stuff from git-introduction repo (e.g. git-concepts) here
Another renaming
README: Replace mailing list with forum, update rules
Add more stuff
More renames
Continue working and moving stuff
Final(?) renaming
Add standarized header and footer navigation
Rebrand the challenges to belong to Scores of Beauty
Rewrite 'setup'
Rewrite 'version control intro'