Add markdown readme for System.Configuration.ConfigurationManager

src/libraries/System.Configuration.ConfigurationManager/src/README.md

@@ -0,0 +1,65 @@
+## About
+
+Provides types that support using XML configuration files (`app.config`). Only use this package when migrating the existing .NET Framework code that already uses XML configuration. When writing a new code, use [Microsoft.Extensions.Configuration](https://www.nuget.org/packages/Microsoft.Extensions.Configuration/) instead.
+
+For more information, see the documentation:
+
+- [Configure apps by using configuration files](https://docs.microsoft.com/dotnet/framework/configure-apps/)
+- [System.Configuration namespace](https://docs.microsoft.com/dotnet/api/system.configuration)
+- [System.Configuration.Configuration](https://docs.microsoft.com/dotnet/api/system.configuration.configuration)
+- [System.Configuration.ConfigurationManager](https://docs.microsoft.com/dotnet/api/system.configuration.configurationmanager)
+
+## Example
+
+The following example shows how to read and modify the application configuration settings.
+
+```cs
+using System;
+using System.Configuration;
+
+class Program
+{
+    static void Main()
+    {
+        try
+        {
+            // Open current application configuration
+            Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
+            KeyValueConfigurationCollection section = config.AppSettings.Settings;
+
+            // Print settings from configuration file
+            foreach (string key in section.AllKeys)
+            {
+                Console.WriteLine($"{key}: {section[key].Value}");
+            }
+
+            // Add new setting
+            section.Add("Database", "TestDatabase");
+
+            // Change existing setting
+            section["Username"].Value = "TestUser";
+
+            // Save changes to file
+            config.Save(ConfigurationSaveMode.Modified);
+            ConfigurationManager.RefreshSection(config.AppSettings.SectionInformation.Name);
+        }
+        catch (ConfigurationErrorsException ex)
+        {
+            Console.WriteLine("Error reading configuration: ");
+            Console.WriteLine(ex.Message);
+        }
+    }
+}
+```
+
+To run this example, include an `app.config` file with the following content in your project:
+
+```xml
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+  <appSettings>
+    <add key="Server" value="example.com"/>
+    <add key="Username" value="Admin"/>
+  </appSettings>
+</configuration>
+```

src/libraries/System.Configuration.ConfigurationManager/src/System.Configuration.ConfigurationManager.csproj

@@ -7,11 +7,8 @@
     <IsTrimmable>false</IsTrimmable>
     <NoWarn>$(NoWarn);CA1847</NoWarn>
     <IsPackable>true</IsPackable>
-    <PackageDescription>Provides types that support using configuration files.
-
-Commonly Used Types:
-System.Configuration.Configuration
-System.Configuration.ConfigurationManager</PackageDescription>
+    <PackageDescription>Provides types that support using XML configuration files (app.config). Only use this package when migrating the existing .NET Framework code that already uses XML configuration. When writing a new code, use Microsoft.Extensions.Configuration instead.</PackageDescription>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>
 
   <!-- DesignTimeBuild requires all the TargetFramework Derived Properties to not be present in the first property group. -->
@@ -278,4 +275,8 @@ System.Configuration.ConfigurationManager</PackageDescription>
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
     <Reference Include="System.Configuration" />
   </ItemGroup>
+
+  <ItemGroup>
+    <None Include="README.md" Pack="true" PackagePath="\"/>
+  </ItemGroup>
 </Project>