[Docs] Add release note for ARM's new -mframe-chain option

This adds a release note entry for the new -mframe-chain option
introduced on D125094.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D129085

llvm/docs/ReleaseNotes.rst

@@ -116,6 +116,8 @@ Changes to the ARM Backend
 * Implemented generation of Windows SEH unwind information.
 * Switched the MinGW target to use SEH instead of DWARF for unwind information.
 * Added support for the Cortex-M85 CPU.
+* Added support for a new -mframe-chain=(none|aapcs|aapcs+leaf) command-line
+  option, which controls the generation of AAPCS-compliant Frame Records.
 
 Changes to the AVR Backend
 --------------------------