[ReleaseNotes] Add note for the new -fexperimental-isel flag.

Patch by Amara Emerson.

Differential Revision: https://reviews.llvm.org/D42860


git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_60@324212 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -132,6 +132,11 @@ New Compiler Flags
   difference between the ``-std=c17`` and ``-std=c11`` language modes is the
   value of the ``__STDC_VERSION__`` macro, as C17 is a bug fix release.
 
+- Added the ``-fexperimental-isel`` and ``-fno-experimental-isel`` flags to
+  enable/disable the new GlobalISel instruction selection framework. This
+  feature is enabled by default for AArch64 at the ``-O0`` optimization level.
+  Support for other targets or optimization levels is currently incomplete.
+
 Deprecated Compiler Flags
 -------------------------