Add xMath (#2591)

README.md

@@ -1216,6 +1216,7 @@ API | Description | Auth | HTTPS | CORS |
 | [USGS Earthquake Hazards Program](https://earthquake.usgs.gov/fdsnws/event/1/) | Earthquakes data real-time | No | Yes | Unknown |
 | [USGS Water Services](https://waterservices.usgs.gov/) | Water quality and level info for rivers and lakes | No | Yes | Unknown |
 | [World Bank](https://datahelpdesk.worldbank.org/knowledgebase/topics/125589) | World Data | No | No | Unknown |
+| [xMath](https://x-math.herokuapp.com/) | Random mathematical expressions | No | Yes | Yes |
 
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