[OpenMP] NFC: Create OpenMP release notes file

cherry-pick: 73b7ff4e16c6c806adca57998cf4144a3be59c49
llvm/llvm-project@73b7ff4

Author: jprotze

docs/ReleaseNotes.rst

@@ -0,0 +1,45 @@
+===========================
+openmp 11.0.0 Release Notes
+===========================
+
+.. contents::
+    :local:
+
+.. warning::
+   These are in-progress notes for the upcoming LLVM 11.0.0 release.
+   Release notes for previous releases can be found on
+   `the Download Page <https://releases.llvm.org/download.html>`_.
+
+Introduction
+============
+
+This document contains the release notes for the OpenMP runtime, release 11.0.0.
+Here we describe the status of openmp, including major improvements
+from the previous release. All openmp releases may be downloaded
+from the `LLVM releases web site <https://llvm.org/releases/>`_.
+
+Non-comprehensive list of changes in this release
+=================================================
+
+5.0 features
+------------
+
+* ...
+
+5.1 features
+------------
+
+* ...
+
+OMPT Improvements
+-----------------
+
+* Added OMPT callbacks for doacross loops, detached tasks
+* Added handling for mutexinoutset dependencies
+
+OMPT-based Tools
+----------------
+
+* Added ompt-multiplex.h as a header-only OMPT-tool to support nesting of OMPT
+  tools. (see openmp/tools/multiplex)
+