Update readme

PiperOrigin-RevId: 270346317

official/nlp/xlnet/README.md

@@ -2,4 +2,15 @@
 
 The academic paper which describes XLNet in detail and provides full results on
 a number of tasks can be found here: https://arxiv.org/abs/1906.08237.
-Instructions and user guide will be added soon.
+
+**Instructions and user guide will be added soon.**
+
+XLNet is a generalized autoregressive BERT-like pretraining language model that
+enables learning bidirectional contexts by maximizing the expected likelihood
+over all permutations of the factorization order. It can learn dependency beyond
+a fixed length without disrupting temporal coherence by using segment-level
+recurrence mechanism and relative positional encoding scheme introduced in
+[Transformer-XL](https://arxiv.org/pdf/1901.02860.pdf). XLNet outperforms BERT
+on 20 NLP benchmark tasks and achieves state-of-the-art results on 18 tasks
+including question answering, natural language inference, sentiment analysis,
+and document ranking.