Add readme for Microsoft.Extensions.Configuration.Xml

src/libraries/Microsoft.Extensions.Configuration.Xml/README.md

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+# Microsoft.Extensions.Configuration.Xml
+
+XML configuration provider implementation for [Microsoft.Extensions.Configuration](https://www.nuget.org/packages/Microsoft.Extensions.Configuration/). This package enables you to read configuration parameters from XML files. You can use [XmlConfigurationExtensions.AddXmlFile](https://learn.microsoft.com/dotnet/api/microsoft.extensions.configuration.xmlconfigurationextensions.addxmlfile) extension method on `IConfigurationBuilder` to add XML configuration provider to the configuration builder.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/core/extensions/configuration-providers#xml-configuration-provider
+
+## Contribution Bar
+- [x] [We consider new features, new APIs, bug fixes, and performance changes](https://github.com/dotnet/runtime/tree/main/src/libraries#contribution-bar)
+
+The APIs and functionality are mature, but do get extended occasionally.
+
+## Deployment
+[Microsoft.Extensions.Configuration.Xml](https://www.nuget.org/packages/Microsoft.Extensions.Configuration.Xml/) is included in the ASP.NET Core shared framework. The package is deployed as out-of-band (OOB) too and can be referenced into projects directly.
+
+## Example
+The following example shows how to read the application configuration from XML file.
+
+```cs
+using System;
+using Microsoft.Extensions.Configuration;
+
+class Program
+{
+    static void Main()
+    {
+        // Build a configuration object from XML file
+        IConfiguration config = new ConfigurationBuilder()
+            .AddXmlFile("appsettings.xml")
+            .Build();
+
+        // Get a configuration section
+        IConfigurationSection section = config.GetSection("Settings");
+
+        // Read simple values
+        Console.WriteLine($"Server: {section["Server"]}");
+        Console.WriteLine($"Database: {section["Database"]}");
+
+        // Read nested values
+        Console.WriteLine($"IPAddress: {section["Endpoint:IPAddress"]}");
+        Console.WriteLine($"Port: {section["Endpoint:Port"]}");
+    }
+}
+```
+
+To run this example, include an `appsettings.xml` file with the following content in your project:
+
+```xml
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+  <Settings>
+    <Server>example.com</Server>
+    <Database>Northwind</Database>
+    <Endpoint>
+      <IPAddress>192.168.0.10</IPAddress>
+      <Port>80</Port>
+    </Endpoint>
+  </Settings>  
+</configuration>
+```
+
+You can include a configuration file using a code like this in your `.csproj` file:
+
+```xml
+<ItemGroup>
+  <Content Include="appsettings.xml">
+    <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+  </Content>
+</ItemGroup>
+```