Emit post-processing 'info' event for Stream-based output

docs/api.md

@@ -27,6 +27,7 @@ The object returned by the constructor implements the
 [stream.Duplex](http://nodejs.org/api/stream.html#stream_class_stream_duplex) class.
 
 JPEG, PNG or WebP format image data can be streamed out from this object.
+When using Stream based output, derived attributes are available from the `info` event.
 
 ```javascript
 sharp('input.jpg')
@@ -37,6 +38,19 @@ sharp('input.jpg')
   });
 ```
 
+```javascript
+// Read image data from readableStream,
+// resize to 300 pixels wide,
+// emit an 'info' event with calculated dimensions
+// and finally write image data to writableStream
+var transformer = sharp()
+  .resize(300)
+  .on('info', function(info) {
+    console.log('Image height is ' + info.height);
+  });
+readableStream.pipe(transformer).pipe(writableStream);
+```
+
 #### metadata([callback])
 
 Fast access to image metadata without decoding any compressed image data.

docs/changelog.md

@@ -10,6 +10,10 @@
   [#338](https://github.com/lovell/sharp/issues/338)
   [@lookfirst](https://github.com/lookfirst)
 
+* Emit post-processing 'info' event for Stream output.
+  [#367](https://github.com/lovell/sharp/issues/367)
+  [@salzhrani](https://github.com/salzhrani)
+
 ### v0.13 - "*mind*"
 
 #### v0.13.1 - 27<sup>th</sup> February 2016