Change cherry-picker's README to treat it as a package

[brettcannon]

If you look at the README for cherry-picker, the instructions tell you to treat it as a module, i.e. python3 -m cherry-picker while in the cherry-picker directory. But everything else about the code wants it to be treated as a package, e.g. __main__.py uses relative imports so you can't do python3 . to execute cherry-picker.

If cherry-picker is going to be a package then I think the instructions should probably be updated to uniformly treat it as such.