[DOCS] Add Release Note for change

Author: Stylie777

llvm/docs/ReleaseNotes.md

@@ -125,6 +125,11 @@ Changes to the ARM Backend
   the required alignment space with a sequence of `0x0` bytes (the requested
   fill value) rather than NOPs.
 
+* When using the `MOVT` or `MOVW` instructions, the Assembler will now check to
+  ensure that any addend that is used is within a 16bit Signed value range. If the
+  addend falls outside of this range, the LLVM backend will emit an error like so
+  `Relocation Not In Range`.
+
 Changes to the AVR Backend
 --------------------------