Update README.md

README.md

@@ -1,7 +1,7 @@
 # omniclone  [![NPM version](https://img.shields.io/npm/v/omniclone.svg)](https://www.npmjs.com/package/omniclone) [![license](https://img.shields.io/github/license/mashape/apistatus.svg)](https://github.com/jfet97/omniclone/blob/master/LICENSE) ![](https://img.shields.io/npm/dt/omniclone.svg)
 An isomorphic and configurable javascript function for object deep cloning.
 ```js
-omniclone(source [, config]);
+omniclone(source [, config, [, visitor]]);
 ```
 
 Example:
@@ -36,6 +36,7 @@ import omniclone from 'omniclone';
 6. similar references are not duplicated
 7. correct cloning of Array objects
 8. correct cloning of RegExp and Date objects
+9. let you define custom cloning logic
 
 ## config
 
@@ -220,6 +221,31 @@ JSON.stringify(source); // '{"a":{"foo":"bar"},"b":{"foo":"bar"}}'
 ```
 When you will use `JSON.parse()`, an `{"foo":"bar"}` object will be created for the `a` prop and a `{"foo":"bar"}` distinct object will be created for the `b` prop. But this is not the initial situation where `source.a == source.b; // true`.
 
+## how to define custom cloning logic?
+You can define a callback function that will be called on each node which copy can be customized:
+```js
+function visitor(node, config) {
+  // do stuff
+}
+```
+The function will receive the `node` and a copy of the `config` object. If the function returns something different than `undefined`, that returned value will become the cloned value. On the contrary if the function returns `undefined` the default cloning algorithm will be used.
+
+You cannot overwrite the default algorithm logic for String, Boolean, Number, Error, Promise, Map, Set, WeakMap, WeakSet, Date and RegExp objects.
+You can overwrite the default algorithm logic for Array, ArrayBuffer, TypedArray, DataView, plain js and custom objects.
+
+Let's see an example where we add custom logic to properly clone a Node.js Buffer object:
+```js
+const buffer = Buffer.from([1, 2, 3, 4]);
+
+const resBuffer = omniclone(buffer, {}, node => {
+  if (node instanceof Buffer) { // it affects only Buffer objects
+    return Buffer.from(node);
+  }
+  return undefined; // all the other node will be cloned as usual
+});
+```
+Thanks to the `instanceof` check, only Buffer objects will be affected by the intervention of the visitor callback.
+
 
 ## warnings and limitations
 1. promises and methods are always copied by reference