Update README.md

README.md

@@ -16,4 +16,14 @@ Mappings of Authority main headings (the first author only) to the related book
 ## License
 
 [Medvik-Books - training dataset](https://huggingface.co/datasets/NLK-NML/Medvik-Books) © 2025 by [National Medical Library](https://nlk.cz) 
-is licensed under [Creative Commons Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/) 
+is licensed under [Creative Commons Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/) 
+
+## Structure
+
+     "text1","text2","category"
+     "Author heading","Doc title","code1|code2"
+
+
+category - multiple values-codes separated by a pipe 
+
+- https://huggingface.co/datasets/NLK-NML/Medvik-Books/blob/main/categories.json