Update README.md

README.md

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 # AudioSet
 
-AudioSet consists of an expanding ontology of 527 audio event classes and a collection of 2M human-labelled 10-second sound clips drawn from YouTube. 
+AudioSet<sup>[1]</sup> consists of an expanding ontology of 527 audio event classes and a collection of 2M human-labelled 10-second sound clips drawn from YouTube. 
 Some clips are missing on YouTube, so the number of files downloaded is different from time to time. 
 This repository contains 20550 / 22160 of the balanced train set, 1913637 / 2041789 of the unbalanced train set (separated into 41 parts), and 18887 / 20371 of the evaluation set.
-The pre-process script can be found at qiuqiangkong's [github](https://github.com/qiuqiangkong/audioset_tagging_cnn).
+The pre-process script can be found at qiuqiangkong's [github](https://github.com/qiuqiangkong/audioset_tagging_cnn)<sup>[2]</sup>.
 
-To improve training efficiency, we add a slightly more balanced subset AudioSet500K.
+To improve training efficiency, we add a slightly more balanced subset AudioSet500K<sup>[3]</sup>.
 
 ## References