Add note on experimental semantic convention implementation, prefix semantics headers with experimental tag

Versioning.md

@@ -65,6 +65,12 @@ Refer to the [ABI Policy](./docs/abi-policy.md) for more details. To summarise:
   allowed to break existing stable interfaces. Feature flags will be removed
   once we have a stable implementation for the signal.
 
+* As an exception, small experimental features in otherwise stable signals/components
+  mayn't necessarily be released under feature flag. These would be flagged as experimental
+  by adding a `NOTE` in it's header file - either at the beginning of file, or as the comment for
+  the experimental API methods. As an example, the Semantic Conventions for
+  trace signal is experimental at the time of the writing, as mentioned [here](https://github.com/lalitb/opentelemetry-cpp/blob/semantic-conv-experimental/api/include/opentelemetry/trace/semantic_conventions.h#L4:L7).
+
 * GitHub releases will be made for all released versions.
 
 ## Example Versioning Lifecycle

api/include/opentelemetry/trace/semantic_conventions.h

@@ -1,6 +1,12 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
+// NOTE:
+// This implementation is based on the experimental specs for trace semantic convention as defined
+// here:
+// https://github.com/open-telemetry/opentelemetry-specification/tree/v1.0.0/specification/trace/semantic_conventions
+// and MAY will change in future.
+
 #pragma once
 
 #include "opentelemetry/common/string_util.h"

sdk/include/opentelemetry/sdk/resource/semantic_conventions.h

@@ -1,3 +1,12 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// NOTE:
+// This implementation is based on the experimental specs for resource semantic convention as
+// defined here:
+// https://github.com/open-telemetry/opentelemetry-specification/tree/v1.0.0/specification/resource/semantic_conventions
+// and MAY will change in future.
+
 #pragma once
 
 #include <type_traits>