Add README.md

Author: eiriktsarpalis

src/Shared/JsonSchemaExporter/README.md

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+# JsonSchemaExporter
+
+Provides a polyfill for the [.NET 9 `JsonSchemaExporter` component](https://learn.microsoft.com/dotnet/standard/serialization/system-text-json/extract-schema) that is compatible with all supported targets using System.Text.Json version 8.
+
+To use this in your project, add the following to your `.csproj` file:
+
+```xml
+<PropertyGroup>
+  <InjectJsonSchemaExporterOnLegacy>true</InjectJsonSchemaExporterOnLegacy>
+</PropertyGroup>
+```