Add markdown readme for System.Configuration.ConfigurationManager (#7…

…2772)

* Update System.Configuration.ConfigurationManager.csproj

Co-authored-by: Dan Moseley <[email protected]>

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

@@ -0,0 +1,65 @@
+## About
+
+Provides types that support using XML configuration files (`app.config`). This package exists only to support migrating existing .NET Framework code that already uses System.Configuration. When writing new code, use another configuration system instead, such as [Microsoft.Extensions.Configuration](https://www.nuget.org/packages/Microsoft.Extensions.Configuration/).
+
+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). This package exists only to support migrating existing .NET Framework code that already uses System.Configuration. When writing new code, use another configuration system instead, such as Microsoft.Extensions.Configuration.</PackageDescription>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>
 
   <!-- DesignTimeBuild requires all the TargetFramework Derived Properties to not be present in the first property group. -->
@@ -292,4 +289,8 @@ System.Configuration.ConfigurationManager</PackageDescription>
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
     <Reference Include="System.Configuration" />
   </ItemGroup>
+
+  <ItemGroup>
+    <None Include="README.md" Pack="true" PackagePath="\"/>
+  </ItemGroup>
 </Project>