[chore][logstransformprocessor] Add note about contrib

Since this processor is experimental and temporary there's no plan
to support it long term. This makes it clear why it's not included
in the contrib distribution releases.

Fixes open-telemetry#29150

processor/logstransformprocessor/README.md

@@ -15,6 +15,7 @@
 <!-- end autogenerated section -->
 
 NOTE - This processor is experimental, with the intention that its functionality will be reimplemented in the [transform processor](../transformprocessor/README.md) in the future.
+For this reason, the processor is not included in the `contrib` distribution.
 
 The logs transform processor can be used to apply [log operators](../../pkg/stanza/docs/operators) to logs coming from any receiver.
 Please refer to [config.go](./config.go) for the config spec.