Add README

opentracing-shim/README.md

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+# OpenTracing Shim
+
+[![Apache License][license-image]][license-image]
+
+The OpenTracing shim is a bridge layer from OpenTelemetry to the OpenTracing API.
+It takes OpenTelemetry Tracer and exposes it as an implementation of an OpenTracing Tracer.
+
+## Usage
+
+Use the TracerShim wherever you initialize your OpenTracing tracers.
+There are 2 ways to expose an OpenTracing Tracer:
+1. From the global OpenTelemetry configuration:
+    ```cpp
+    auto tracer_shim = TracerShim::createTracerShim();
+    ```
+1. From a provided OpenTelemetry Tracer instance:
+    ```cpp
+    auto tracer_shim = TracerShim::createTracerShim(tracer);
+    ```
+
+Optionally, you can specify propagators to be used for the OpenTracing `TextMap` and `HttpHeaders` formats:
+
+```cpp
+OpenTracingPropagators propagators{
+  .text_map = nostd::shared_ptr<TextMapPropagator>(new trace::propagation::JaegerPropagator()),
+  .http_headers = nostd::shared_ptr<TextMapPropagator>(new trace::propagation::HttpTraceContext())
+};
+
+auto tracer_shim = TracerShim::createTracerShim(tracer, propagators);
+```
+
+If propagators are not specified, OpenTelemetry's global propagator will be used.
+
+## License
+
+Apache 2.0 - See [LICENSE][license-url] for more information.
+
+## Useful links
+
+- For more information on OpenTelemetry, visit: <https://opentelemetry.io/>
+- For help or feedback on this project, join us in [GitHub Discussions][discussions-url]
+
+[discussions-url]: https://github.com/open-telemetry/opentelemetry-cpp/discussions
+[license-url]: https://github.com/open-telemetry/opentelemetry-cpp/blob/main/LICENSE
+[license-image]: https://img.shields.io/badge/license-Apache_2.0-green.svg?style=flat