docs: v3.0.0 changelog

Author: kozistr

docs/changelogs/v3.0.0.md

@@ -4,6 +4,8 @@ Major version is updated! (`v2.12.0` -> `v3.0.0`) (#164)
 
 ### Feature
 
+* Implement `REX` lr scheduler. (#217, #222)
+  * [Revisiting Budgeted Training with an Improved Schedule](https://arxiv.org/abs/2107.04197) 
 * Implement `Aida` optimizer. (#220, #221)
   * [A DNN Optimizer that Improves over AdaBelief by Suppression of the Adaptive Stepsize Range](https://arxiv.org/abs/2203.13273)
 * Implement `WSAM` optimizer. (#213, #216)