add c# server reflection howto

doc/csharp/server_reflection.md

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+# gRPC C# Server Reflection
+
+This document shows how to use gRPC Server Reflection in gRPC C#.
+Please see [C++ Server Reflection Tutorial](../server_reflection_tutorial.md)
+for general information and more examples how to use server reflection.
+
+## Enable server reflection in C# servers
+
+C# Server Reflection is an add-on library.
+To use it, first install the [Grpc.Reflection](https://www.nuget.org/packages/Grpc.Reflection/)
+Nuget package into your project.
+
+Unlike in other languages, with C# you need to manually register the service
+descriptors with the reflection service implementation when creating a server.
+```csharp
+// the reflection service will be aware of "Greeter" and "ServerReflection" services.
+var reflectionServiceImpl = new ReflectionServiceImpl(Greeter.Descriptor, ServerReflection.Descriptor);
+server = new Server()
+{
+    Services =
+    {
+        // the server will serve 2 services, the Greeter and the ServerReflection
+        ServerReflection.BindService(new GreeterImpl()),
+        ServerReflection.BindService(reflectionServiceImpl)
+    },
+    Ports = { { "localhost", 50051, ServerCredentials.Insecure } }
+};
+server.Start();
+```
+
+After starting the server, you can verify that the server reflection
+is working properly by using the `grpc_cli` command line tool:
+
+ ```sh
+  $ grpc_cli ls localhost:50051
+  ```
+
+  output:
+  ```sh
+  helloworld.Greeter
+  grpc.reflection.v1alpha.ServerReflection
+  ```
+
+  For more examples and instructions how to use the `grpc_cli` tool,
+  please refer to the [`grpc_cli` documentation](../command_line_tool.md)
+  and the [C++ Server Reflection Tutorial](../server_reflection_tutorial.md).
+
+## Additional Resources
+
+The [Server Reflection Protocol](../server-reflection.md) provides detailed
+information about how the server reflection works and describes the server reflection
+protocol in detail.