README.md for Extension directory (#569)

Author: AFFogarty

src/csharp/Extensions/README.md

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+# .Net for Apache Spark Extensions
+
+## Table of Contents
+* [NuGet Packages](#nuget-packages)
+
+## NuGet Packages
+
+The following .Net for Apache Spark extensions are available as NuGet packages:
+
+### First-Party
+
+* [Microsoft.Spark.Extensions.Azure.Synapse.Analytics](https://www.nuget.org/packages/Microsoft.Spark.Extensions.Azure.Synapse.Analytics/)
+* [Microsoft.Spark.Extensions.Delta](https://www.nuget.org/packages/Microsoft.Spark.Extensions.Delta/)
+* [Microsoft.Spark.Extensions.DotNet.Interactive](https://www.nuget.org/packages/Microsoft.Spark.Extensions.DotNet.Interactive/)
+* [Microsoft.Spark.Extensions.Hyperspace](https://www.nuget.org/packages/Microsoft.Spark.Extensions.Hyperspace/)
+
+### Third-Party
+
+* Community-created extensions can be added here.