update ReleaseNotes.md

Author: punkyc

llvm/docs/ReleaseNotes.md

@@ -201,6 +201,7 @@ Changes to the RISC-V Backend
 * Adds experimental assembler support for the SiFive Xsfmm* Attached Matrix
   Extensions.
 * `-mcpu=andes-a25` and `-mcpu=andes-ax25` were added.
+* The `Shlcofideleg` extension was added.
 
 Changes to the WebAssembly Backend
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