revise prepare ucf101

data_tools/ucf101/PREPARING_UCF101.md

@@ -23,7 +23,13 @@ If you have some SSD, then we recommend extracting frames there for better I/O p
 mkdir /mnt/SSD/ucf101_extracted/
 ln -s /mnt/SSD/ucf101_extracted/ ../data/ucf101/rawframes
 ```
-Afterwards, run the following script to extract frames.
+
+If you didn't install dense_flow in the installation or only want to play with RGB frames (since extracting optical flow can be both time-comsuming and space-hogging), consider running the following script to extract **RGB-only** frames.
+```shell
+bash extract_rgb_frames.sh
+```
+
+If both rgb and optical flow are required, run the following script to extract frames alternatively.
 ```shell
 bash extract_frames.sh
 ```