Add README.md

Author: Spikhalskiy

temporal-opentracing/README.md

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+# Temporal [OpenTracing](https://opentracing.io/) support module
+
+This module provides a set of Interceptors that adds support for OpenTracing Span Context propagation to Temporal.
+
+## Usage
+
+You want to register two interceptors - one on the Temporal client side, another on the worker side:
+
+1. Client configuration:
+    ```java
+    WorkflowClientOptions.newBuilder()
+       //...
+       .setInterceptors(new OpenTracingClientInterceptor())
+       .build();
+    ```
+2. Worker configuration:
+    ```java
+    WorkerFactoryOptions.newBuilder()
+       //...
+       .setWorkerInterceptors(new OpenTracingWorkerInterceptor())
+       .build();
+    ```
+
+## [OpenTelemetry](https://opentelemetry.io/)
+
+OpenTracing has been merged into OpenTelemetry and nowadays OpenTelemetry should be a preferred solution.
+There is still plenty of OpenTracing usage everywhere and there is an official OpenTracing -> OpenTelemetry bridge,
+but no OpenTelemetry -> OpenTracing bridges.
+
+To give the best coverage in the simplest way, this module is implemented based on OpenTracing for now.
+OpenTelemetry users are advised to use the 
+[OpenTracing -> OpenTelemetry bridge](https://github.com/open-telemetry/opentelemetry-java/tree/main/opentracing-shim) 
+to hook their OpenTelemetry setup and make it available for OpenTracing API:
+
+```java
+    Tracer tracer = OpenTracingShim.createTracerShim();
+    //Tracer tracer = OpenTracingShim.createTracerShim(openTelemetry);
+    GlobalTracer.registerIfAbsent(tracer);
+```
+
+