Update install instruction for pip

[DenisScherbakov]

A wheel package is published on PyPi and on pypi.anaconda.org.
README is part of wheel metadata which is published on PyPi and this update is needed to specify pypi.anaconda.org as recommended installation channel.

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• Have you added a test, reproducer or referred to issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• If this PR is a work in progress, are you filing the PR as a draft?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/index.html

[antonwolfy]

Thanks a lot for updating the instruction, @DenisScherbakov LGTM!