Add a note about caching and docker to the readme

[Earlopain]

By default the RuboCop cache directory is inside a users home directory (cache path docs).

When developing inside docker this path isn't persisted by default, meaning that when invocing rubocop inside the docker container the cache is discarded every time.

In the context of a Rails application there already exists a directory that the user should be persisting (either through a volume or just by mounting it in from the host), the tmp directory in the Rails application root. This folder contains, among other things, development secrets which should be persisted across runs so that things like sessions are still valid after a container restart.

I originally wanted to make this part of the default config but I realized that this gem isn't specific to Rails applications. So I think a mention in the readme will do instead.

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