Readme

Work in progress for comments -- don't merge yet. 

There are a few things missing from the readme that can affect people who are just trying out Pillow for the first time. 

1) There's no installation instructions, and the PIL ones are out dated.
2) There's no complete list of dependencies, since we've added a few
3) The change in the import statements is the lone incompatibility that I'm aware of, but it's a big one. 

I'm going to continue to work on this, including testing out the distro specific information and merging more of the old PIL 1.1.7 documentation.