Add CONTRIBUTING

[losingkeys]

Link to things like:

• How to run a development version of this plugin locally (for vim-plug you just specify the path: https://github.com/junegunn/vim-plug#example, include an example for pathogen and no plugin manager too)
• Add a CoC and link to it
• Add info about commit messages
  • http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
  • http://mislav.net/2014/02/hidden-documentation/
  • Split changes up into small logical commits, each commit should be able to stand on its own
• Formatting (match what's there)
• Other stuff?