Add Accelerated Computing Hub to CuPy references

[leofang]

ACH (Accelerated Computing Hub, https://github.com/NVIDIA/accelerated-computing-hub) is where we at NVIDIA publish CUDA C++ and Python tutorials, workshop and course materials. @brycelelbach @nv-kriehl and others have been leading many well-received workshops, teaching about how to use CuPy and other packages to accelerate Python programs. This should be a useful resource.

[kmaehashi]

Sounds good, maybe it's better to split the References section in README and create sections like Learning Materials.