Add PFN to the README
CuPy consists of the core multi-dimensional array class, `cupy.ndarray`, and many functions on it.
It supports a subset of `numpy.ndarray` interface.
+CuPy is being developed by [Preferred Networks Inc.](https://preferred.jp/en/)
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## Installation
For detailed instructions on installing CuPy, see [the installation guide](https://docs-cupy.chainer.org/en/stable/install.html).
