add a README for wheels

[jameslamb]

Wheel-building CI jobs are failing like this:

Checking final_dist/cuvs_cu12-25.2.0a26-cp310-cp310-manylinux_2_28_aarch64.whl: FAILED due to warnings
WARNING long_description missing.
Error: Process completed with exit code 1.

(build link)

Looks like the root cause is a combination of the following:

• there was a new twine release (6.x) 3 days ago: https://pypi.org/project/twine/#history
• it contains Fix #908 pypa/twine#1168, which makes twine check --strict fail if the wheel's long_description is empty
• the cuvs wheel README (used as the wheel long_description) is empty

This proposes adding a small README, with just 2 sentences copied from the project's root-level README, to get past that check.

Notes for Reviewers

The long_description becomes the project homepage when a project is hosted on PyPI. The wheels produced from this repo aren't currently being published to pypi.org so this change won't be seen there, but a more user-friendly README should be added if/when we decide to publish cuvs-cu{11,12} to pypi.org.

ref: rapidsai/build-planning#70