Update README.md

[GhadimiR]

Description:
To maintain state, this action uses GitHub Actions Cache, via the API. One of the operations it needs to perform in the event of a cache conflict is to delete the existing cache, and this requires the actions: write permission.

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull Request Overview

This PR updates the README to include the required actions: write permission for deleting existing caches via the GitHub Actions Cache API.

• Adds actions: write under the permissions block in the example workflow.

[ericLemanissier]

Alternatively, #1237 could be considered, avoiding unnecessary priviledge elevation

[GhadimiR]

Thanks for weighing in @ericLemanissier, I've had a look over your PR and while I agree that there's some merit to the idea of using unique cache keys to just generate new caches instead of resolving a conflict, that isn't the kind of thing we'd do lightly. Would prefer to make this small change to fix the docs and consider something like that for a future major release.