Add release note in "Potentially Breaking Changes"

Author: naveen-seth

clang/docs/ReleaseNotes.rst

@@ -69,6 +69,9 @@ Potentially Breaking Changes
   call the member ``operator delete`` instead of the expected global
   delete operator. The old behavior is retained under ``-fclang-abi-compat=21``
   flag.
+- Downstream projects that previously linked only against ``clangDriver`` may
+  now (also) need to link against the new ``clangOptions`` library, since
+  options-related code has been moved out of the Driver into a separate library.
 
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------