[docs] Add Deprecated section to ReleaseNotes

This is brought up in https://reviews.llvm.org/D83915.
We would like to remove some feature in PowerPC.

We did send RFC before, but we think it might be a better idea that
we indicate planned removal in the Release Notes for version 11
and actual removal in those for version 12..

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D83968

Author: Jinsong Ji

llvm/docs/ReleaseNotes.rst

@@ -31,6 +31,10 @@ LLVM web page, this document applies to the *next* release, not the current
 one.  To see the release notes for a specific release, please see the `releases
 page <https://llvm.org/releases/>`_.
 
+Deprecated and Removed Features/APIs
+=================================================
+* BG/Q support, including QPX, will be removed in the 12.0.0 release.
+
 Non-comprehensive list of changes in this release
 =================================================
 .. NOTE