CONTRIBUTING.md: Start listing maintainers

Also suggest using commit message and pull request summary prefixes,
along the lines of those used by the Linux and Git projects.

Maintainers should link to their GitHub profile, and I've used a blank
line to separate these maintainer links from the other links used in
the text.  Alphabetical order would be nice, but it doesn't really
matter ;).
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