Document cache location

[Columbus240]

The documentation does not say, where the cache files are stored. From current src/platform.rs I see, that dirs::cache_dir is used for that, which specifies the location. It would have been helpful, to have a note somewhere in the documentation about the cache location.

[weilbith]

Furthermore, it would be great to allow for customization. I like that is uses the XDG base directories. However, I like to separate caches per project.