Add Node - pnpm example

[karlhorky]

Description

Add an example of caching pnpm, from the last version in the pnpm readme

Motivation and Context

1. There is no current example for pnpm
2. The documentation for using actions/cache with pnpm has now been removed from the pnpm readme

How Has This Been Tested?

This has been tested extensively by the pnpm community, over years

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:

• [x] 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.
• [ ] All new and existing tests passed.