readme: move "choosing an api" to its own main section
readme: move "choosing an api" to its own main section
indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard