Add Frankfurter (#948)

README.md

@@ -222,6 +222,7 @@ API | Description | Auth | HTTPS | CORS |
 | [ExchangeRate-API](https://www.exchangerate-api.com) | Free currency conversion | No | Yes | Yes |
 | [Exchangeratesapi.io](https://exchangeratesapi.io) | Exchange rates with currency conversion | No | Yes | Yes |
 | [Fixer.io](http://fixer.io) | Exchange rates and currency conversion | `apiKey` | Yes | Unknown |
+| [Frankfurter](https://www.frankfurter.app/docs) | Exchange rates, currency conversion and time series | No | Yes | Yes |
 | [ratesapi](https://ratesapi.io) | Free exchange rates and historical rates | No | Yes | Unknown |
 
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