docs: document skill version pinning via git refs

[yunaremaia]

Summary

Adds a "Pinning a version" section to the README Installation docs, answering the versioning question raised in #48.

What's documented

• Install a specific skill version by appending a git ref (#ref) to the package path — commit SHA, tag, or branch:

  npx skills add google/skills/skills/cloud/gke-basics#6f0b877
  npx skills add google/skills/skills/cloud/gke-basics#main

• Same ref syntax works with Microsoft's apm (Agent Package Manager), as demonstrated in Versioning #48:

  apm install -g google/skills/skills/cloud/gke-basics#6f0b877

• Use case note: reproducible agent setups (CI pipelines, team workspaces).

Verification

• npx skills add google/skills/skills/cloud/gke-basics#main was run and correctly resolved to Source: https://github.com/google/skills.git @ main (skills/cloud/gke-basics)
• Ref syntax is the same one demonstrated with apm in issue Versioning #48

Closes #48

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