Add instructions for running locally

[VeckoTheGecko]

I found that it is really easy to run this locally using a uv one liner.

FYI in Parcels-code/Parcels#2435 we're bypassing the GitHub action (since its nice for users who want to reproduce locally to just copy the command, rather than rely on CI - which I think is really beneficial for a workflow like this). Not sure if you want to highlight - this way of using the tool - further up the README

e.g.,

jobs:
  min-version-policy:
    name: Minimum Version Policy
    runs-on: "ubuntu-latest"
    env:
      COLUMNS: 120
    steps:
      - uses: actions/checkout@v5
      - uses: astral-sh/setup-uv@v7
      - run: |
          uv run --with 'minimum-dependency-versions @ git+https://github.com/xarray-contrib/[email protected]' \
          python -m minimum_versions validate \
          --policy=.github/ci/policy.yaml \
          --manifest-path=./pixi.toml \
          pixi:test-minimum

(PS: I'm going offline for a while (~1week) so may not respond - feel free to take over this PR)