Add a copyright and license comment to re2.py
Add a copyright and license comment to re2.py

This is to prepare for adding some not-totally-trivial code to it.

andreasvc/pyre2,simudream/pyre2,facebook/pyre2,facebook/pyre2,pombredanne/pyre2,pombredanne/pyre2,sunu/pyre2,sunu/pyre2,andreasvc/pyre2,pombredanne/pyre2,simudream/pyre2,axiak/pyre2,axiak/pyre2