[ReleaseNotes] Add SystemZ target section

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_60@324726 91177308-0d34-0410-b5e6-96231b3b80d8

Author: uweigand

docs/ReleaseNotes.rst

@@ -120,6 +120,15 @@ Changes to the PowerPC Target
 
  During this release ...
 
+Changes to the SystemZ Target
+-----------------------------
+
+During this release the SystemZ target has:
+
+* Added support for 128-bit atomic operations.
+
+* Added support for the "o" constraint for inline asm statements.
+
 Changes to the X86 Target
 -------------------------