doc: add io.js LICENSE block

PR-URL: https://github.com/iojs/io.js/pull/294


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>

Discussed in TC meeting 2015-01-13, merge but accept changes
if better advice comes along