Add a note about readable in flowing mode

doc/api/stream.markdown

@@ -164,6 +164,8 @@ readable.on('readable', function() {
 Once the internal buffer is drained, a `readable` event will fire
 again when more data is available.
 
+The `readable` event is not emitted in the "flowing" mode with the sole exception of the last one, on end-of-stream.
+
 #### Event: 'data'
 
 * `chunk` {Buffer | String} The chunk of data.
@@ -182,6 +184,8 @@ readable.on('data', function(chunk) {
 })
 ```
 
+Note that `'readable'` event should not be used together with `'data'`, because the assigning the latter switches the stream into the "flowing" mode, so the `'readable'` event will not emitted.
+
 #### Event: 'end'
 
 This event fires when there will be no more data to read.