This will allow subsequent actions to get access to the HTML URL for the
release created with this. Handy for composing multiple actions together
that are related to the release.

In my case I wanted to get the  URL into a slack message posted to the
team when a release is published.

The output can be referenced by using the `steps.release.ouput.url` in
the workflow yaml:

    - name: Release
      id: release
      uses: softprops/action-gh-release@v1
      with:
        name: "My Release"
      env:
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
    - name: Slack
      uses: csexton/slack-message-action@v1
      with:
        message: New release posted at ${{ steps.release.outputs.url}}