Update CHANGELOG.md

CHANGELOG.md

@@ -70,6 +70,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added `Pytorch Geometric` integration example with Lightning ([#4568](https://github.com/PyTorchLightning/pytorch-lightning/pull/4568))
 
 
+- Added `Trainer.validate()` method to perform one evaluation epoch over the validation set (
+    [#4707](https://github.com/PyTorchLightning/pytorch-lightning/pull/4707))
+
+
 ### Changed
 
 - Tuner algorithms will be skipped if `fast_dev_run=True` ([#3903](https://github.com/PyTorchLightning/pytorch-lightning/pull/3903))