Add another way to verify ccache in CONTRIBUTING.md (#49337)

Summary:
In the case people are confused how to make sure ccache is working, I added another sentence in the documentation for how to check that the symlinks are correctly set up in addition to waiting for 2 clean builds of PyTorch.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/49337

Reviewed By: walterddr

Differential Revision: D25535659

Pulled By: janeyx99

fbshipit-source-id: 435696255f517c074dd0d9f96534d22b60f795b2