Add opentopodata (#1328)

README.md

@@ -521,6 +521,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Mexico](https://github.com/IcaliaLabs/sepomex) | Mexico RESTful zip codes API | No | Yes | Unknown |
 | [One Map, Singapore](https://docs.onemap.sg/) | Singapore Land Authority REST API services for Singapore addresses | `apiKey` | Yes | Unknown |
 | [OnWater](https://onwater.io/) | Determine if a lat/lon is on water or land | No | Yes | Unknown |
+| [Open Topo Data](https://www.opentopodata.org) | Elevation and ocean depth for a latitude and longitude | No | Yes | No |
 | [OpenCage](https://opencagedata.com) | Forward and reverse geocoding using open data | `apiKey` | Yes | Yes |
 | [openrouteservice.org](https://openrouteservice.org/) | Directions, POIs, isochrones, geocoding (+reverse), elevation, and more | `apiKey` | Yes | Unknown |
 | [OpenStreetMap](http://wiki.openstreetmap.org/wiki/API) | Navigation, geolocation and geographical data | `OAuth` | No | Unknown |