contributing: Transition from GitHub's wiki format to pure Markdown
Merge branch 'restructured-version-control' into restructured

* restructured-version-control:
  version-control/git/README.md: Add paragraph pitching Git
  version-control/README.md: Add a motivational paragraph
  version-control/git: Use git-and-github notes
  version-control/svn/cheet-sheet.md: Add a Subversion-specific cheat sheet
  version-control/git/cheet-sheet.md: Add a Git-specific cheat sheet
  version-control/cheet-sheet.md: Add a generic outline for the cheat sheet
  version-control: Make README.md and instructor.md more generic
  version-control: Add README.md and instructor.md from the guide