Document bitmap transparency and public API

Explain image behavior for each credential tile version, package the README and XML documentation, and complete the public API comments.

src/samples/Lithnet.CredentialProvider.Sample.Core.x64/TestCredentialProviderCoreX64.cs

@@ -101,6 +101,7 @@ namespace Lithnet.CredentialProvider.Samples
 
         private static Bitmap CreateTransparentUserTile()
         {
+            // CredentialTile3 preserves the alpha channel in this image. CredentialTile and CredentialTile2 render it against the control's BackgroundColor.
             Bitmap image = new Bitmap(128, 128, PixelFormat.Format32bppArgb);
 
             using (Graphics graphics = Graphics.FromImage(image))

src/samples/Lithnet.CredentialProvider.Sample.Framework.x64/TestCredentialProviderTile.cs

@@ -5,6 +5,9 @@ using Microsoft.Extensions.Logging;
 
 namespace Lithnet.CredentialProvider.Samples
 {
+    /// <summary>
+    /// Demonstrates a version 3 credential tile that preserves image transparency.
+    /// </summary>
     public class TestCredentialProviderTile : CredentialTile3
     {
         private TextboxControl UsernameControl;