docs: v3.1.0 changelog

Author: kozistr

docs/changelogs/v3.1.0.md

@@ -10,6 +10,7 @@
 * Support more bnb optimizers. (#258)
     * `bnb_paged_adam8bit`, `bnb_paged_adamw8bit`, `bnb_*_*32bit`.
 * Improve `power_iteration()` speed up to 40%. (#259)
+* Improve `reg_noise()` (E-MCMC) speed up to 120%. (#260)
 
 ### Refactor