policy: added contribution policy

Policy originally by @isaacs and @othiym23, submitted by @mikeal,
committed fresh by @rvagg because .. #22 is kind of a mess

PR-URL: https://github.com/iojs/io.js/pull/22
Reviewed-By: Mikeal Rogers <mikeal.rogers@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Rod Vagg <rod@vagg.org>