An editor-only static class that adds a menu item to the Editor menu. When clicked it shows a simple dialog saying "It worked!" and a brief message.
using Editor;
public static class MyEditorMenu
{
[Menu( "Editor", "Simple Multiplayer Tool/My Menu Option" )]
public static void OpenMyMenu()
{
EditorUtility.DisplayDialog( "It worked!", "This is being called from your library's editor code!" );
}
}