Mention that restore/save paths much match

[timbaverstock-bmbl]

Description

Add a line pointing out that the restore and save operations' paths must match.

Motivation and Context

I fell down a rabbit hole expecting to be able to save and restore to different locations. Even in the absence of clearer error messages, this line in the README would have helped.

https://github.com/actions/cache/issues/1653

How Has This Been Tested?

I read it?

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ x ] Documentation (add or update README or docs)

Checklist:

• [ ] My code follows the code style of this project.
• [ x ] My change requires a change to the documentation.
• [ x ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.