Separate README in repo from README on PyPI

[lintool]

See https://github.com/castorini/pyserini/blob/master/setup.py#L3

I think it's a good idea to keep the README on PyPI separate from (and simpler than) the README in the repo. That way, we also don't have weird references to the repo itself in the repo README (so that you can point the PyPI README to the repo).