Document usage in a merge queue

[AntonNiklasson]

I'm using this action in a merge queue. I'm not sure I've gotten all the details right just yet, but I think it works.

Merge queues are still in beta: https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

The main issue I ran into was that there was no git repository available by default, like you get on the pull_request event. I'm currently doing this:

  files-changed:
    runs-on: [ self-hosted ]
    outputs:
      any: ${{ steps.changes.outputs.any }}
    steps:
      - uses: actions/checkout@v3
      - uses: dorny/paths-filter@v2
        id: changes
        with:
          base: ${{ github.event.merge_group.base_ref}}
          ref: ${{ github.event.merge_group.head_ref }}
          filters: |
            ui:
              - 'ui/**'

[saschabratton]

@AntonNiklasson thanks!

Setting explicit base and ref is perfectly fine solution, but we won't document this specific use case for it. Instead, we should add default handling for merge queues. There is a pending PR for this here: #255