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/Lithnet.CredentialProvider/Controls/ComboboxControl.cs

@@ -10,13 +10,13 @@ namespace Lithnet.CredentialProvider
         private int selectedItemIndex;
 
         /// <summary>
-        /// Creates a new <c ref="ComboboxControl"/> control
+        /// Creates a new <see cref="ComboboxControl"/> control.
         /// </summary>
         /// <param name="key">The unique key for this control</param>
         public ComboboxControl(string key) : this(key, null) { }
 
         /// <summary>
-        /// Creates a new <c ref="ComboboxControl"/> control
+        /// Creates a new <see cref="ComboboxControl"/> control.
         /// </summary>
         /// <param name="key">The unique key for this control</param>
         /// <param name="label">The label associated with the control</param>
@@ -107,4 +107,4 @@ namespace Lithnet.CredentialProvider
             return clone;
         }
     }
-}
+}