how to use the `ccache` package on Ubuntu (#18495)

Summary:
Added full instructions for how to use the `ccache` package. Thanks.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18495

Differential Revision: D14635351

Pulled By: ezyang

fbshipit-source-id: 158e1052bae580e95f73644252fdbddcc0213128