Add CHANGELOG.

Author: Oberon00

CHANGELOG.md

@@ -22,6 +22,10 @@ release.
 
 ### Semantic Conventions
 
+- BREAKING: Change enum member IDs to lowercase without spaces, not starting with numbers.
+  Change values of `net.host.connection.subtype` to match.
+  ([#1863](https://github.com/open-telemetry/opentelemetry-specification/pull/1863))
+
 ### Compatibility
 
 ### OpenTelemetry Protocol