Setup 1.9.x release branch

[bwbarrett]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bwbarrett]

The RELEASENOTES.md is a copy of the HEAD of the v1.8.x-aws branch, with the 1.9.0 block added:

% git diff v1.8.x-aws v1.9.x-aws-setup -- RELEASENOTES.md                                                                                                                                                                   ✭
diff --git a/RELEASENOTES.md b/RELEASENOTES.md
index 9a566a8..cbccf40 100644
--- a/RELEASENOTES.md
+++ b/RELEASENOTES.md
@@ -11,6 +11,25 @@ AWS-specific parts a compile-time option.  When a feature (or entire
 release) was only available in one of the two variants, we note that
 in the release notes.
 
+# v1.9.0-aws release notes
+
+New Features:
+* Support v8 plugin interface introduced with NCCL 2.20.  This enables
+  the use of the user memory registration feature recently introduced
+  in NCCL.
+* Reduce ordering constraints for control messages, to reduce head of
+  line blocking under congestion.
+
+Bug Fixes:
+* Increase the number of communicators to 256K (from 1K), supporting larger
+  all-to-all groups.
+* Improve logging in some corner case error conditions.
+
+The plugin has been tested with following libfabric providers using tests
+bundled in the source code and
+[nccl-tests](https://github.com/NVIDIA/nccl-tests) suite:
+* efa
+
 # v1.8.1-aws release notes
 This is a bugfix release that requires
 [Libfabric v1.18.0](https://github.com/ofiwg/libfabric/releases/tag/v1.18.0) or later and