Update README.md

Author: rm-code

README.md

@@ -62,3 +62,28 @@ Or by using the ```setAnchor``` function:
 -- Invert anchor status
 node:setAnchor( not node:isAnchor(), mouseX, mouseY )
 ```
+
+### Using custom classes for Nodes and Edges
+
+If you prefer to not touch the default classes, you can simply inherit from them and tell Graphoon to use your custom classes instead.
+
+```lua
+local GraphLibraryNode = require('lib.libfdgraph.fd').Node
+
+local CustomNodeClass = {}
+
+-- You can pass additional arguments to your custom class. Just make sure the
+-- default parameters ar in the right order.
+function CustomNodeClass.new( id, x, y, anchor, ... )
+    local self = GraphLibraryNode.new( id, x, y, anchor )
+
+	-- ... Custom code
+end
+
+return CustomNodeClass
+```
+
+```lua
+local GraphLibrary = require('Graphoon').Graph
+GraphLibrary.setNodeClass( require('CustomNodeClass') )
+```