Add NBP exchange rates API

Add NBP (National Bank of Poland) currency exchange rates API

README.md

@@ -231,6 +231,7 @@ API | Description | Auth | HTTPS | CORS |
 | [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 |
+| [National Bank of Poland](http://api.nbp.pl/en.html) | A collection of currency exchange rates (data in XML and JSON) | No | Yes | Yes |
 | [ratesapi](https://ratesapi.io) | Free exchange rates and historical rates | No | Yes | Unknown |
 | [VATComply.com](https://www.vatcomply.com/documentation) | Exchange rates, geolocation and VAT number validation | No | Yes | Yes |