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.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

This README.md describes it as yaml.

Suggested change

	```yml
	```yaml