add some docs info

docs/source/Recipes.rst

@@ -73,14 +73,24 @@ Simple transcoding
 
 To transcode a video, you have to pass the target format to FFMpeg.
 
+When you define a format which implements the
+:ref:`Resizable <resizable-reference>` interface.
+You must set FFprobe (see :ref:`FF-probe<ffprobe-reference>`)
+for probing the media and found its height and size.
+
 The following example initialize a Ogg format and encodes a `Video.mpeg` to a
 target file `file.ogv` :
 
 .. code-block:: php
 
     <?php
+    use FFMpeg\FFProbe;
     use FFMpeg\Format\Video\Ogg;
 
+    $ffprobe = FFProbe::load($logger);
+
+    $ffmpeg->setProber($ffprobe);
+
     $oggFormat = new Ogg();
 
     $ffmpeg->open('Video.mpeg')
@@ -178,6 +188,8 @@ Advanced media type
 This section presents usage for the different interfaces. You can combine
 them for your own formats.
 
+.. _resizable-reference:
+
 Resizable
 .........
 
@@ -207,13 +219,15 @@ The example below resizes a video by half.
         ->encode($format, 'file.mp4')
         ->close();
 
+.. _resamplable-reference:
 
 Resamplable
 ...........
 
 This interface provides video resampling. The example below resample the video
 at 15 frame per second with a I-frame every 30 image (see
-`GOP on wikipedia <https://wikipedia.org/wiki/Group_of_pictures>`_).
+`GOP on wikipedia <https://wikipedia.org/wiki/Group_of_pictures>`_) and supports
+B-frames (see `B-frames on wikipedia <https://wikipedia.org/wiki/Video_compression_picture_types>`_)
 
 .. code-block:: php
 
@@ -234,6 +248,10 @@ at 15 frame per second with a I-frame every 30 image (see
             return 30;
         }
 
+        public function supportBFrames()
+        {
+            return true;
+        }
     }
 
     $format = new MyFormat();
@@ -477,6 +495,8 @@ latest AvConv / FFMPeg version, aac encoding has to be executed with extra comma
         ->encode($format, 'output-aac.mp4')
         ->close();
 
+.. _ffprobe-reference:
+
 FFProbe recipes
 ---------------