Add a CONTRIBUTING.md doc for potential helpers

CONTRIBUTING file shows potential contributers how to get the code
checked out and running and explains expectations for
code/tests/documentation.

github "display[s] a link to the CONTRIBUTING file when a user creates
an Issue or opens a Pull Request" see
https://github.com/blog/1184-contributing-guidelines
Merge pull request #492 from spazm/feature/add_CONTRIBUTING_file

Add a CONTRIBUTING.md doc for potential helpers