Add 'clone' method to snapshot an instance

Cloned instances share a common input
Allows multiple output Streams to use a single input Stream

README.md

@@ -163,6 +163,15 @@ var pipeline = sharp()
 readableStream.pipe(pipeline);
 ```
 
+```javascript
+var pipeline = sharp().rotate();
+pipeline.clone().resize(800, 600).pipe(firstWritableStream);
+pipeline.clone().extract(20, 20, 100, 100).pipe(secondWritableStream);
+readableStream.pipe(pipeline);
+// firstWritableStream receives auto-rotated, resized readableStream
+// secondWritableStream receives auto-rotated, extracted region of readableStream
+```
+
 ```javascript
 sharp('input.png')
   .rotate(180)
@@ -347,6 +356,14 @@ Do not process input images where the number of pixels (width * height) exceeds
 
 `pixels` is the integral Number of pixels, with a value between 1 and the default 268402689 (0x3FFF * 0x3FFF).
 
+#### clone()
+
+Takes a "snapshot" of the instance, returning a new instance.
+Cloned instances inherit the input of their parent instance.
+This allows multiple output Streams
+and therefore multiple processing pipelines
+to share a single input Stream.
+
 ### Image transformation options
 
 #### resize(width, [height])