Added 3RD_PARTY.md (confluentinc#1696)

Author: Matt Howlett

3RD_PARTY.md

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+
+## 3rd Party Libraries
+
+A (very incomplete) list of libraries that make use of or expand on the capability of Confluent.Kafka. These libraries are not supported by Confluent Inc.
+
+To add your project, open a pull request!
+
+- [Streamiz Kafka .NET](https://github.com/LGouellec/kafka-streams-dotnet) - .NET Stream Processing Library for Apache Kafka.
+- [Chr.Avro](https://github.com/ch-robinson/dotnet-avro) - A modern and flexible Avro implementation for .NET. Integrates seamlessly with Confluent.Kafka and Schema Registry.
+- [Multi Schema Avro Deserializer](https://github.com/ycherkes/multi-schema-avro-desrializer) - Avro deserializer for reading messages serialized with multiple schemas.
+- [OpenSleigh.Transport.Kafka](https://github.com/mizrael/OpenSleigh/tree/develop/src/OpenSleigh.Transport.Kafka) - A Kafka Transport for OpenSleigh, a distributed saga management library.

README.md

@@ -250,6 +250,11 @@ All `Consume` errors will result in a `ConsumeException` with further informatio
 available via the `Error` and `ConsumeResult` fields.
 
 
+### 3rd Party
+
+There are numerous libraries that expand on the capabilities provided by Confluent.Kafka, or use Confluent.Kafka
+to integrate with Kafka. For more information, refer to the [3rd Party Libraries](3RD_PARTY.md) page.
+
 ### Confluent Cloud
 
 For a step-by-step guide on using the .NET client with Confluent Cloud see [Getting Started with Apache Kafka and .NET](https://developer.confluent.io/get-started/dotnet/) on [Confluent Developer](https://developer.confluent.io/).