CONTRIBUTING.md: update commit formatting suggestions
CONTRIBUTING.md: update commit formatting suggestions

- link to a decent in-depth explanation of git formatting and why it
  matters

- update to reflect best practice for title: 50 (max 72) characters

- remove example of committing full description from command-line. It's
  best practice to wrap messages at 72 characters (plays much nicer on
  e.g. git log), and that's near-impossible to do well from the
  command-line. I'd rather let users rely on their own editors at this
  stage. Fine if there's a single title line though.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3