Adds a CONTRIBUTING file

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

* mostly modeled on riemann.io/howto.html
* adds link for lien install/usage documentation
* expands lien target documentation
* adds link to github pull request usage
* missing a link for "clojure client" repo.