Add changelog entry

Author: tigrannajaryan

CHANGELOG.md

@@ -23,6 +23,7 @@ release.
 - Fix the inconsistent formatting of semantic convention enums ([#1598](https://github.com/open-telemetry/opentelemetry-specification/pull/1598/))
 - Add details for filling resource for AWS Lambda([#1610](https://github.com/open-telemetry/opentelemetry-specification/pull/1610))
 - Add already specified `messaging.rabbitmq.routing_key` span attribute key to the respective YAML file.
+- Clarify usage of "otel." attribute namespace ([#1640](https://github.com/open-telemetry/opentelemetry-specification/pull/1640)).
 
 ### Compatibility