[docs] Update release notes with items related to Flang

Differential Revision: https://reviews.llvm.org/D109317

Author: banach-space

llvm/docs/ReleaseNotes.rst

@@ -55,6 +55,8 @@ Non-comprehensive list of changes in this release
  emits valid unwind entrypoints which are validated when the context is being
  set during exception handling.
 
+* Flang is now included in the binary packages released by LLVM.
+
 Changes to the LLVM IR
 ----------------------