Update Conda description in PyTorch README (#20726)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20726
Edward says it doesn't actually provide compilers,
but it does provide dependencies, so let's mention that instead.
Reviewed By: ezyang
Differential Revision: D15423316
fbshipit-source-id: 9b384f88e5bf7a3d2c132508620c276b49e1569f
