Add a release note about the --build-id change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271918 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -47,7 +47,10 @@ sections with improvements to Clang's support for those languages.
 Major New Features
 ------------------
 
-- Feature1...
+- Clang will no longer passes --build-id by default to the linker. In modern
+  linkers that is a relatively expensive option. It can be passed explicitly
+  with -Wl,--build-id. To have clang always pass it, build it with
+  -DENABLE_LINKER_BUILD_ID.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^