Updated description of the lookup-only input for main action

[kotewar]

Description

Updated readme to give users more clarity on newly added input lookup-only in the cache action.

Motivation and Context

The variable naming was hard since its used in both actions - restore and cache, hence the readme files need clarity on the working of the inputs.

How Has This Been Tested?

NA, readme change only.

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