Update README.md

README.md

@@ -55,3 +55,5 @@ The following data fields are available:
 - **abstract**: abstract of the document.
 - **keyphrases**: list of reference keyphrases.
 - **prmu**: list of <u>P</u>resent-<u>R</u>eordered-<u>M</u>ixed-<u>U</u>nseen categories for reference keyphrases.
+
+**NB**: The present keyphrases (represented by the "P" label in the PRMU column) are sorted by their apparition order in the text (title + abstract).