Fix issue #613 and issue #614

When trying to install `SimpleCV` on my Mac OS X 10.9, I mainly got two issues:

*    [brew doesn't have PIL as a formula](https://github.com/sightmachine/SimpleCV/issues/613)

*   [fatal error: 'freetype/fterrors.h' file not found when building PIL on Mac OS X](https://github.com/sightmachine/SimpleCV/issues/614)

So I do some fix and improvement to the `README` file.