Add contributing.md

Having read a few bits of advice about Contributor Agreements, also
known as Contributor License Agreements (CLA), it seemed sensible to
compromise on a halfway house: adding a `contributing.md` file that will
advise potential contributors, and which will be shown to them if they
submit a pull request via GitHub (where this project's repository is
currently hosted). Some references:
http://www.tldp.org/HOWTO/Software-Release-Practice-HOWTO/licensetheory.html
http://www.tldp.org/HOWTO/Software-Release-Practice-HOWTO/licensepractice.html
https://en.wikipedia.org/w/index.php?title=Contributor_License_Agreement&oldid=622809877
http://programmers.stackexchange.com/q/191535
http://web.archive.org/web/20131221163944/http://jayunit.net/2013/01/09/clahub-easy-contributor-agreements-on-github/
http://web.archive.org/web/20131201192450/http://jacobian.org/writing/contributor-license-agreements/
http://www.groklaw.net/article.php?story=20110524120303815
http://opensource.com/law/10/9/copyright-aggregation
https://fossbazaar.org/content/open-source-contributor-agreements-purpose-and-scope/
https://github.com/blog/1184-contributing-guidelines
https://help.github.com/articles/setting-guidelines-for-repository-contributors