Update vocabularies.rst

docs/vocabularies.rst

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 Vocabularies and model
 ======================
 
-The model used in Senpy is based on the following vocabularies:
+The model used in Senpy is based on NIF 2.0 [1], which defines a semantic format and API for improving interoperability among natural language processing services.
 
-* Marl, a vocabulary designed to annotate and describe subjetive opinions expressed on the web or in information systems.
-* Onyx, which is built one the same principles as Marl to annotate and describe emotions, and provides interoperability with Emotion Markup Language.
-* NIF 2.0, which defines a semantic format and APO for improving interoperability among natural language processing services
+Senpy has been applied to sentiment and emotion analysis services using the following vocabularies:
+
+* Marl [2,6], a vocabulary designed to annotate and describe subjetive opinions expressed on the web or in information systems.
+* Onyx [3,5], which is built one the same principles as Marl to annotate and describe emotions, and provides interoperability with Emotion Markup Language.
+
+An overview of the vocabularies and their use can be found in [4].
+
+
+[1] Guidelines for developing NIF-based NLP services, Final Community Group Report 22 December 2015https://www.w3.org/2015/09/bpmlod-reports/nif-based-nlp-webservices/
+[2] Marl http://www.gsi.dit.upm.es/ontologies/marl/
+[3] Onyx http://www.gsi.dit.upm.es/ontologies/onyx/
+[4] Iglesias, C. A., Sánchez-Rada, J. F., Vulcu, G., & Buitelaar, P. (2017). Linked Data Models for Sentiment and Emotion Analysis in Social Networks. In Sentiment Analysis in Social Networks (pp. 49-69).
+[5] Sánchez-Rada, J. F., & Iglesias, C. A. (2016). Onyx: A linked data approach to emotion representation. Information Processing & Management, 52(1), 99-114.
+[6] Westerski, A., Iglesias Fernandez, C. A., & Tapia Rico, F. (2011). Linked opinions: Describing sentiments on the structured web of data.