Allow integrations to specify an application documentation link

The release PR body contains a link to this repo, but an integration that wraps this library (e.g. the [GitHub app](https://github.com/googleapis/repo-automation-bots/tree/master/packages/release-please) or [GitHub action](https://github.com/google-github-actions/release-please-action)) should be able to add a link to its documentation.
