Add nodes without edges

[andreasgrosche]

The graph is populated using Edges as in the example from the README:

public static Graph MyGraph
{
  get
  {
    var graph = new Graph();
    graph.Edges.Add(new Edge("A", "B"));
    graph.Edges.Add(new Edge("A", "D"));
    graph.Edges.Add(new Edge("A", "E"));
    graph.Edges.Add(new Edge("B", "C"));
    graph.Edges.Add(new Edge("B", "D"));
    graph.Edges.Add(new Edge("D", "A"));
    graph.Edges.Add(new Edge("D", "E"));
    return graph;
  }
}

How to add single "floating" nodes that do not have edges?