Provide example for coveralls in README

[MarcoIeni]

Motivation

Upload to coveralls example might be useful for someone.

Workflow example

This is an example taken from here.

      - name: Run cargo-tarpaulin
        uses: actions-rs/[email protected]
        with:
          args: '--ignore-tests --out Lcov'

      - name: upload to Coveralls
        uses: coverallsapp/github-action@master
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          path-to-lcov: './lcov.info'