add readme

sdk/trace/internal/x/README.md

@@ -0,0 +1,33 @@
+# Experimental Features
+
+The Trace SDK contains features that have not yet stabilized.
+These features are added to the OpenTelemetry Go Trace SDK prior to
+stabilization so that users can start experimenting with them and provide
+feedback.
+
+These feature may change in backwards incompatible ways as feedback is applied.
+See the [Compatibility and Stability](#compatibility-and-stability) section for
+more information.
+
+## Features
+
+- [OnEnding Processor](#onending-processor)
+
+### OnEnding Processor
+
+Processor implementations sometimes want to be able to modify a span after it
+ended, but before it becomes immutable.
+A processor that implements the `OnEnding` method can use that callback to
+perform such modifications.
+
+It can be used to implement tail-based sampling for example.
+
+## Compatibility and Stability
+
+Experimental features do not fall within the scope of the OpenTelemetry Go
+versioning and stability [policy](../../../../VERSIONING.md).
+These features may be removed or modified in successive version releases,
+including patch versions.
+
+When an experimental feature is promoted to a stable feature, a migration path
+will be included in the changelog entry of the release.