docs: update README to include recent improvements in validation loss calculation

Author: kohya-ss

README.md

@@ -14,6 +14,11 @@ The command to install PyTorch is as follows:
 
 ### Recent Updates
 
+Feb 26, 2025:
+
+- Improve the validation loss calculation in `train_network.py`, `sdxl_train_network.py`, `flux_train_network.py`, and `sd3_train_network.py`. PR [#1903](https://github.com/kohya-ss/sd-scripts/pull/1903)
+  - The validation loss uses the fixed timestep sampling and the fixed random seed. This is to ensure that the validation loss is not fluctuated by the random values.
+
 Jan 25, 2025:
 
 - `train_network.py`, `sdxl_train_network.py`, `flux_train_network.py`, and `sd3_train_network.py` now support validation loss. PR [#1864](https://github.com/kohya-ss/sd-scripts/pull/1864) Thank you to rockerBOO!