Add citation in README (open-mmlab#995)

Author: zhouzaida

CITATION.cff

@@ -0,0 +1,8 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - name: "MMEngine Contributors"
+title: "OpenMMLab Foundational Library for Training Deep Learning Models"
+date-released: 2022-09-01
+url: "https://github.com/open-mmlab/mmengine"
+license: Apache-2.0

README.md

@@ -308,6 +308,19 @@ runner.train()
 
 We appreciate all contributions to improve MMEngine. Please refer to [CONTRIBUTING.md](CONTRIBUTING.md) for the contributing guideline.
 
+## Citation
+
+If you find this project useful in your research, please consider cite:
+
+```
+@article{mmengine2022,
+  title   = {{MMEngine}: OpenMMLab Foundational Library for Training Deep Learning Models},
+  author  = {MMEngine Contributors},
+  howpublished = {\url{https://github.com/open-mmlab/mmengine}},
+  year={2022}
+}
+```
+
 ## License
 
 This project is released under the [Apache 2.0 license](LICENSE).

README_zh-CN.md

@@ -294,6 +294,19 @@ runner.train()
 
 我们感谢所有的贡献者为改进和提升 MMEngine 所作出的努力。请参考[贡献指南](CONTRIBUTING_zh-CN.md)来了解参与项目贡献的相关指引。
 
+## 引用
+
+如果您觉得 MMEngine 对您的研究有所帮助，请考虑引用它：
+
+```
+@article{mmengine2022,
+  title   = {{MMEngine}: OpenMMLab Foundational Library for Training Deep Learning Models},
+  author  = {MMEngine Contributors},
+  howpublished = {\url{https://github.com/open-mmlab/mmengine}},
+  year={2022}
+}
+```
+
 ## 开源许可证
 
 该项目采用 [Apache 2.0 license](LICENSE) 开源许可证。