add some exposition to mpu example in README.md

This is re @mikeal's comment here:
#345 (comment)

This updates the mpu example in the README with some more comments and a callback to make the usage unmistakably clear for beginners and especially forgetful developers (e.g. @mikermcneil)

README.md

@@ -108,12 +108,20 @@ request.post('http://service.com/upload').form({key:'value'})
 For `multipart/form-data` we use the [form-data](https://github.com/felixge/node-form-data) library by [@felixge](https://github.com/felixge). You don’t need to worry about piping the form object or setting the headers, `request` will handle that for you.
 
 ```javascript
-var r = request.post('http://service.com/upload')
+var r = request.post('http://service.com/upload', function optionalCallback (err, httpResponse, body) {
+  if (err) {
+    return console.error('upload failed:', err);
+  }
+  console.log('Upload successful!  Server responded with:', body);
+})
 var form = r.form()
 form.append('my_field', 'my_value')
 form.append('my_buffer', new Buffer([1, 2, 3]))
 form.append('my_file', fs.createReadStream(path.join(__dirname, 'doodle.png'))
 form.append('remote_file', request('http://google.com/doodle.png'))
+
+// Just like always, `r` is a writable stream, and can be used as such (you have until nextTick to pipe it, etc.)
+// Alternatively, you can use the callback (that's what this example does-- see `optionalCallback` above).
 ```
 
 ## HTTP Authentication