Add some minimal release notes for retpoline support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_60@326539 91177308-0d34-0410-b5e6-96231b3b80d8

Author: chandlerc

docs/ReleaseNotes.rst

@@ -42,6 +42,12 @@ Non-comprehensive list of changes in this release
 * Preliminary support for Sanitizers and sibling features on X86(_64) NetBSD
   (ASan, UBsan, TSan, MSan, SafeStack, libFuzzer).
 
+* Support for `retpolines <https://support.google.com/faqs/answer/7625886>`_
+  was added to help mitigate "branch target injection" (variant #2) of the
+  "Spectre" speculative side channels described by `Project Zero
+  <https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html>`_
+  and the `Spectre paper <https://spectreattack.com/spectre.pdf>`_.
+
 
 Changes to the LLVM IR
 ----------------------
@@ -183,6 +189,10 @@ During this release the X86 target has:
 
 * Improved documentation for SSE/AVX intrinsics in intrin.h header files.
 
+* Gained support for emitting `retpolines
+  <https://support.google.com/faqs/answer/7625886>`_, including automatic
+  insertion of the necessary thunks or using external thunks.
+
 
 External Open Source Projects Using LLVM 6
 ==========================================