Update readme for pre-releases as the checkout needs ref: sha

[sbe-arg]

in order for pre-release branch validation to work its needs to be clear that the workflow checkout requires the sha

      - uses: actions/checkout@v3
        with:
          ref: ${{ github.sha }}
          fetch-depth: 0

non having the sha in some scenarios resolves to main, master with a the default RELEASE_BRANCHES array resulting in a always pre-release set to false