Added documentation for the watermark filter (#221)

Added documentation for the watermark filter

README.md

@@ -191,6 +191,35 @@ The resize filter takes three parameters :
 - `$mode`, one of the constants `FFMpeg\Filters\Video\ResizeFilter::RESIZEMODE_*` constants
 - `$useStandards`, a boolean to force the use of the nearest aspect ratio standard.
 
+###### Watermark
+
+Watermark a video with a given image.
+
+```php
+$video
+    ->filters()
+    ->watermark($watermarkPath, array(
+        'position' => 'relative',
+        'bottom' => 50,
+        'right' => 50,
+    ));
+```
+
+The watermark filter takes two parameters:
+
+`$watermarkPath`, the path to your watermark file.
+`$coordinates`, an array defining how you want your watermark positioned. You can use relative positioning as demonstrated above or absolute as such:
+
+```php
+$video
+    ->filters()
+    ->watermark($watermarkPath, array(
+        'position' => 'absolute',
+        'x' => 1180,
+        'y' => 620,
+    ));
+```
+
 ###### Framerate
 
 Changes the frame rate of the video.