Add links to generated documentation in README

[wangela]

The README for this repo lists the APIs and provides a link to Postman, but if I would like to browse rendered documentation it would be helpful to know where I can see the rendered docs.

Perhaps adding a column to the table in https://github.com/googlemaps/openapi-specification/blob/main/README.md#description with links to the corresponding pages in https://developers.google.com/maps/documentation/ would be the most straightforward way to do this.