Update README.md

Updates the description of PyTorch correlation to note that it is required for training (see yoxu515#48)

README.md

@@ -42,7 +42,7 @@ General examples (Messi and Kobe):
    * pytorch >= 1.7.0 and torchvision
    * opencv-python
    * Pillow
-   * Pytorch Correlation (Recommend to install from [source](https://github.com/ClementPinard/Pytorch-Correlation-extension) instead of using `pip`. **The project can also work without this module but will lose some efficiency of the short-term attention**.)
+   * Pytorch Correlation (Recommend to install from [source](https://github.com/ClementPinard/Pytorch-Correlation-extension) instead of using `pip`. **The project can also work without this module for inference but will lose some efficiency of the short-term attention. It is required for training.**.)
 
 Optional:
    * scikit-image (if you want to run our **Demo**, please install)