Update README with OpenCensus sunset notice (#2111)

README.md

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+> **Warning**
+>
+> OpenCensus and OpenTracing have merged to form [OpenTelemetry](https://opentelemetry.io), which serves as the next major version of OpenCensus and OpenTracing.
+>
+> OpenTelemetry has now reached feature parity with OpenCensus, with tracing and metrics SDKs available in .NET, Golang, Java, NodeJS, and Python. **All OpenCensus Github repositories, except [census-instrumentation/opencensus-python](https://github.com/census-instrumentation/opencensus-python), will be archived on July 31st, 2023**. We encourage users to migrate to OpenTelemetry by this date.
+>
+> To help you gradually migrate your instrumentation to OpenTelemetry, bridges are available in Java, Go, Python, and JS. [**Read the full blog post to learn more**](https://opentelemetry.io/blog/2023/sunsetting-opencensus/).
+
 # OpenCensus - A stats collection and distributed tracing framework
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