update readme to 1.8.0

update readme

Fix for Issue NVIDIA#115 (https://github.com/NVlabs/cub/issues/115)

WarpReduce segmented reduction broken in CUDA 9 for logical warp sizes <
32

Fix for NVIDIA#110 (https://github.com/NVlabs/cub/issues/110) DeviceH…

…istogram

null-pointer exception bug for iterator inputs

- Update device histogram testing to include iterator-based samples
- Prevent a few macro redefinitions
- Update doc for 1.7.2

1.7.2 changelog

Update README for 1.7.1

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