add README

I added a `code` tag to find the model easily among other code models, and a small description of the model.

README.md

@@ -1,3 +1,26 @@
----
-license: apache-2.0
----
+---
+language: code
+license: apache-2.0
+
+---
+# UniXcoder
+
+## Model description
+
+UniXcoder is a unified cross-modal pre-trained model for programming languages to support both code-related understanding and generation tasks.
+[UniXcoder: Unified Cross-Modal Pre-training for Code Representation.](https://arxiv.org/abs/2203.03850) Daya Guo, Shuai Lu, Nan Duan, Yanlin Wang, Ming Zhou, Jian Yin.
+[GitHub](hhttps://github.com/microsoft/CodeBERT/tree/master/UniXcoder#unixcoder)
+
+## Citation
+
+If you use this code or UniXcoder, please consider citing the following paper:
+
+```
+@article{guo2022unixcoder,
+  title={UniXcoder: Unified Cross-Modal Pre-training for Code Representation},
+  author={Guo, Daya and Lu, Shuai and Duan, Nan and Wang, Yanlin and Zhou, Ming and Yin, Jian},
+  journal={arXiv preprint arXiv:2203.03850},
+  year={2022}
+}
+```
+