add note for gulp 4 in readme

readme.markdown

@@ -2,8 +2,7 @@
 
 [Streams](http://nodejs.org/api/stream.html "Stream") are node's best and most misunderstood idea, and EventStream is a toolkit to make creating and working with streams easy.
 
-Normally, streams are only used for IO, but in event stream we send all kinds of objects down the pipe. If your application's input and output are streams,  
-shouldn't the throughput be a stream too?  
+Normally, streams are only used for IO, but in event stream we send all kinds of objects down the pipe. If your application's input and output are streams,  shouldn't the throughput be a stream too?  
 
 The *EventStream* functions resemble the array functions, because Streams are like Arrays, but laid out in time, rather than in memory.  
 
@@ -13,6 +12,8 @@ All the `event-stream` functions return instances of `Stream`.
 
 >NOTE: I shall use the term <em>"through stream"</em> to refer to a stream that is writable <em>and</em> readable.  
 
+>NOTE for Gulp users: Merge will not work for gulp 4. [merge-stream](https://npmjs.com/merge-stream) should be used. 
+
 ### [simple example](https://github.com/dominictarr/event-stream/blob/master/examples/pretty.js):
 
 ``` js