Add release notes

.release-notes/4547.md

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+##  Add support for pinning actors to a dedicated scheduler thread
+
+Pony programmers can now pin actors to a dedicated  scheduler thread. This can be required/used for interfacing with C libraries that rely on thread local storage. A common example of this is graphics/windowing libraries.
+
+The way it works is that an actor can request that it be pinned and then check to confirm that the pinning was successfully applied after which all subsequent behaviors on that actor will run on the same scheduler thread until the actor is destroyed or the actor requests to be unpinned.
+
+Additional details can be found in the `actor_pinning` package in the standard library.