[repo] Add ReleaseNotes.md to include features shipped in stable release

RELEASENOTES.md

@@ -0,0 +1,15 @@
+# RELEASE NOTES
+
+This file contains highlights and announcements covering all components.
+For more details see `CHANGELOG.md` files maintained in the root source
+directory of each individual package.
+
+## 1.9.0
+
+* `Exemplars` are now part of the stable API! For details see: [customizing
+  exemplars
+  collection](https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/docs/metrics/customizing-the-sdk#exemplars).
+
+* `WithLogging` is now part of the stable API! Logging, Metrics, and Tracing can
+  now all be configured using the `With` style and the builders finally have
+  parity in their APIs.

src/OpenTelemetry/CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+This file contains individual changes for the OpenTelemetry package.
+For highlights and announcements covering all components see: [Release Notes](../../RELEASENOTES.md).
+
 ## Unreleased
 
 * Added `OpenTelemetrySdk.Create` API for configuring OpenTelemetry .NET signals