Added release note about OpenTelemetry

In particular, highlighting that it is not "phone home" telemetry.

docs/releases/1.29-NOTES.md

@@ -6,6 +6,25 @@ This is a document to gather the release notes prior to the release.
 
 # Significant changes
 
+## Initial OpenTelemetry Support
+
+We are starting to add (experimental) support for OpenTelemetry,
+in particular Tracing support.  Setting `OTEL_EXPORTER_OTLP_TRACES_FILE`
+will write a trace file which can then be read by the traceserver program.
+More information and options are described in [docs/opentelemetry.md](/docs/opentelemetry.md).
+The tracing data is not expected to be particularly useful for end-users in
+this release; the (non-standard) recording approach is instead intended to
+work well with our Prow end-to-end testing system so that developers can
+optimize kOps.
+
+Please note: this is *not* telemetry in the "phone-home" sense.
+The kOps project does not collect data from your machine.  As an
+open-source project we do not even want to collect any of your data.
+Currently the only OpenTelemetry backend supported is writing to a
+filesystem (and it is opt-in).  In future you will be able to configure
+other OpenTelemetry backends, but this data will only be sent if
+you enable OpenTelemetry, and only sent to where you configure.
+
 ## AWS
 
 * Network Load Balancers in front of the Kubernetes API and bastion hosts now