[DOC] libcudf requirements are out of date in CONTRIBUTING.md

[harrism]

https://github.com/rapidsai/cudf/blob/branch-0.19/CONTRIBUTING.md reports

Get libcudf Dependencies

Compiler requirements:

    gcc version 5.4+
    nvcc version 10.0+
    cmake version 3.14.0+

CUDA/GPU requirements:

    CUDA 10.0+
    NVIDIA driver 410.48+
    Pascal architecture or better

This should be updated. Ideally, it should be updated in a way that does not require changes every release. Is there a more central requirements list that is kept up to date that we can just link to?

CC @kkraus14

[harrism]

Fixed by #7778.