Add loud warnings to encourage nvidia-ml-py migration

[rjzamora]

There are no plans to maintain the (unofficial) pynvml package. This PR is an attempt to encourage faster user migration to the official nvidia-ml-py package:

• It adds a new warning to the top of the README file
• It adds a FutureWarning when pynvml.smi is imported, since an equivalent module does not exist in nvidia-ml-py

@kenhester - I would prefer for the pynvml.smi warning to point the user somewhere else for smi support. Let me know if you (or anyone else) are motivated to create and maintain such space. Unfortunately, there is no change for that space to be within nvidia-ml-py.

[rjzamora]

Update...@pentschev pointed out in an offline discussion that it will be difficult for all RAPIDS-related projects to migrate to nvidia-ml-py in a coordinated way (which would be necessary since both packages are imported as pynvml). He suggested that we convert this repo to a "meta package" that simply adds nvidia-ml-py as a dependency. If we are taking the time to refactor the project in this way, it could also make sense to move the smi bindings into a distinct pynvml_utils module before the pynvml module is removed.

[rjzamora]

Merging the README change, and moving the smi refactor to a new PR.

[pentschev]

Thanks Rick!