Added note on Python 3 and Pillow.

Pillow versions 2.0.0 and later support Python 3.2 and 3.3, whereas [PIL](http://www.pythonware.com/products/pil/) does not support any Python 3 release.

Pillow's [PyPi page](https://pypi.python.org/pypi/Pillow/2.1.0):
> Pillow < 2.0.0 supports Python versions 2.4, 2.5, 2.6, 2.7; Pillow >= 2.0.0 supports Python versions 2.6, 2.7, 3.2, 3.3.

PIL's [homepage]:
> The current free version is PIL 1.1.7. This release supports Python 1.5.2 and newer, including 2.5 and 2.6. A version for 3.X will be released later.