Update README.md with instructions re: local.py

Otherwise it fails with:

    ImportError: Could not import settings 'anthropod.settings.dev' (Is it on sys.path?): No module named local

If you remove that line, though, it still fails with:

    ImproperlyConfigured: The SECRET_KEY setting must not be empty.

So I add an instruction that creates local.py and defines SECRET_KEY randomly.