[PPC] Added a note to release notes

A draft line added to release notes for PPC, to keep a record of changes.
This is just a draft and will be rewritten towards the end of release.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265694 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Ehsan Amiri

docs/ReleaseNotes.rst

@@ -97,7 +97,7 @@ Changes to the MIPS Target
 Changes to the PowerPC Target
 -----------------------------
 
- During this release ...
+ Moved some optimizations from O3 to O2 (D18562)
 
 
 Changes to the X86 Target