Added link to specification

Author: AP-G

README.md

@@ -6,6 +6,9 @@ The JSON Schema NDR TS defines a standardised method to generate the UN/CEFACT d
 
 This repository serves as a test platform during the project phase. Subsequently, the final location for the publication of the UN/CEFACT standards will be specified. The generated JSON schema files will serve as standardised input for the JSON-LD project from the end of this project. In this way, conformity between these two formats will be ensured. 
 
+The [JSON schema NDR technical specification](https://uncefact.unece.org/download/attachments/124026918/API-TECH-SPEC_JSON_Schema_NDR.pdf?api=v2) from the public review can be found [here](https://uncefact.unece.org/download/attachments/124026918/API-TECH-SPEC_JSON_Schema_NDR.pdf?api=v2).
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 ## Content
 
 In this repository there are two main publications of UN/CEFACT JSON schema artefacts: