Update release notes after #2431 merge

Signed-off-by: Ganyu (Bruce) Xu <g66xu@uwaterloo.ca>

Author: xuganyu96

RELEASE.md

@@ -55,6 +55,9 @@ SPHINCS+ was removed in 0.16.0.
 [25d0a9e](https://github.com/open-quantum-safe/liboqs/commit/25d0a9e)
 - Fixed incorrect array size when calling `secure_clean`
 [ab0e07c](https://github.com/open-quantum-safe/liboqs/commit/ab0e07c)
+- Implemented optimization barrier `OQS_MEM_BLACK_BOX` and applied to
+`ct_select` in FrodoKEM
+[#2431](https://github.com/open-quantum-safe/liboqs/pull/2431)
 
 ### Significant change