Added credits/link to official EDIFACT pages from UNECE

Author: nerdoc

README.md

@@ -148,6 +148,8 @@ There are some additional tests to check the performance of parsing huge files -
 make test-extended
 ```
 
+## Credits
+The official formats for UN/EDIFAT provided by UN Secretariat are raw text files located [here](http://www.unece.org/tradewelcome/un-centre-for-trade-facilitation-and-e-business-uncefact/outputs/standards/unedifact/directories/download.html) and html (.htm) files located [here](http://www.unece.org/tradewelcome/un-centre-for-trade-facilitation-and-e-business-uncefact/outputs/standards/unedifact/directories/2011-present.html), similarly for service codes as text and html files located at https://www.gefeg.com/jswg/.
 
 
 ## License