Add Ultra (#173)

* Add Ultra

* Update README.md

---------

Co-authored-by: Kevin Kandlbinder <kevin@kevink.dev>

Author: dschep

README.md

@@ -99,6 +99,7 @@ We also have a list of <a href="UNMAINTAINED.md">unmaintained projects</a>. If y
 * [ohsome Dashboard](https://ohsome.org/apps/dashboard/) - Tool for visualizing changes over a certain time period. ([Wiki](https://wiki.openstreetmap.org/wiki/Ohsome_Dashboard))
 * [OSM Statistics](https://piebro.github.io/openstreetmap-statistics/) - Up-to-date OSM statistics about editor usage, corporate contributions and more.
 * [Bellingcat OpenStreetMap search](https://osm-search.bellingcat.com/) - Web based tool to find geolocation leads by searching for proximate features on OpenStreetMap. ([Source Code](https://github.com/bellingcat/osm-search) / [Article](https://www.bellingcat.com/resources/how-tos/2023/05/08/finding-geolocation-leads-with-bellingcats-openstreetmap-search-tool/))
+* [Ultra](https://overpass-ultra.us/) - Web based tool for making maps with a variety of data APIs such as overpass, postpass, qlever, ohsome, sophox, and more. ([Source Code](https://gitlab.com/trailstash/ultra))
 
 ### Mobile Tools