add cmkd preprint info

README.md

@@ -13,6 +13,8 @@
 
 ## News
 
+March, 2022: We released a new preprint [*CMKD: CNN/Transformer-Based Cross-Model Knowledge Distillation for Audio Classification*](https://arxiv.org/abs/2203.06760), where we proposed a knowledge distillation based method to further improve the AST model performance without changing its architecture.
+
 Feb, 2022: The [Self-Supervised AST (SSAST)](https://arxiv.org/abs/2110.09784) code is released [[**here**]](https://github.com/YuanGongND/ssast). SSAST use self-supervised pretraining instead of supervised ImageNet pretraining, so it supports arbitrary patch shape and size (e.g., a temperal frame and a square patch) with a good performance.
 
 Nov, 2021: The [PSLA training pipeline](https://arxiv.org/abs/2102.01243) used to train AST and baseline efficientnet model code is released [[**here**]](https://github.com/YuanGongND/psla). It is a strong audio classification training pipeline that can be used for most deep learning models. Also, it has a one-click FSD50K recipe that achieves SOTA 0.567 mAP.