add info about custom attributes

ffplayout/filters/README.md

@@ -7,3 +7,18 @@ Add your one filters here. They must have the correct file naming:
 The file itself should contain only one filter in a function named `def filter(prope):`
 
 Check **v_addtext.py** for example.
+
+In your filter you can also read custom properties from the current program node. That you can use for any usecase you wish, like reading a subtitle file, or a different logo for every clip and so on.
+
+The normal program node looks like:
+
+```JSON
+{
+    "in": 0,
+    "out": 3600.162,
+    "duration": 3600.162,
+    "source": "/dir/input.mp4"
+}
+```
+
+This you can extend to your needs, and apply this values to your filters.