Add changelog

Author: landonxjames

.changelog/unknown-variants.md

@@ -0,0 +1,10 @@
+---
+applies_to: ["client", "server"]
+authors: ["rcoh"]
+references: ["smithy-rs#4132"]
+breaking: false
+new_feature: false
+bug_fix: true
+---
+
+Smithy unions that contain members named "unknown" will now codegen correctly