[Snek] [keith-packard/snek] acbbbf: Add contributing guide

[Keith Packard]

  Branch: refs/heads/master
  Home:   https://github.com/keith-packard/snek
  Commit: acbbbf81da5b3efcddbfaa86e0db9f7695cddda8
      https://github.com/keith-packard/snek/commit/acbbbf81da5b3efcddbfaa86e0db9f7695cddda8
  Author: Keith Packard <keithp at keithp.com>
  Date:   2019-06-10 (Mon, 10 Jun 2019)

  Changed paths:
    A CONTRIBUTING.md

  Log Message:
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  Add contributing guide