In README.md CMAKE_PREFIX_PATH should be CONDA_PREFIX when using an conda virtual...

In README.md CMAKE_PREFIX_PATH should be CONDA_PREFIX when using an conda virtual environment (#15548)

Summary:
In current README.md, `CMAKE_PREFIX_PATH` is set to conda root even when you have activated an virtual environment. When an conda virtualenv is activated, packages are installed in `CONDA_PREFIX`, not conda root. I think `CMAKE_PREFIX_PATH` should also be set to `CONDA_PREFIX` in this case. I think some build issues can be solved with the new instruction. Maybe something like #14954.

soumith,
When I made PR #15335 I was confused and made a wrong point. I think this PR could be the real solution.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15548

Differential Revision: D13549681

Pulled By: soumith

fbshipit-source-id: 42d855b6e49ee58d735d2f4715d3e5752a748693