Add note on EU Open Data Portal vs GISCO data source choice

Author: michalosmenda

README.md

@@ -558,6 +558,8 @@ No Python code changes are required.
 
 [GISCO TERCET flat files](https://ec.europa.eu/eurostat/web/gisco/geodata/administrative-units/postal-codes) ([download](https://gisco-services.ec.europa.eu/tercet/flat-files)), © European Union – GISCO, licensed [CC-BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).
 
+The [EU Open Data Portal dataset](https://data.europa.eu/data/datasets/postcodes-and-nuts-nomenclature-of-territorial-units-for-statistics) was also considered as a data source. However, its refresh cycle lags behind the GISCO TERCET flat files, so direct sourcing from GISCO was chosen for more up-to-date coverage.
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 ## Licence
 
 Licensed under the [European Union Public Licence (EUPL) v. 1.2](LICENCE).