spelling and add docstring

ffplayout/filter/README.md

@@ -1,9 +1,9 @@
 # Custom Filters
 
-Sdd your one filters here. They must have the correct file naming:
+Add your one filters here. They must have the correct file naming:
 - for audio filter: a_[filter name].py
 - for video filter: v_[filter name].py
 
-The file it self should contain only one filter in a function named `def filter(prope):`
+The file itself should contain only one filter in a function named `def filter(prope, type):`
 
 Check **v_addtext.py** for example.

ffplayout/filter/v_drawtext.py

@@ -5,6 +5,9 @@ from ffplayout.utils import _text
 
 
 def filter(probe):
+    """
+    extract title from file name and overlay it
+    """
     font = ''
     source = os.path.basename(probe.src)
     match = re.match(_text.regex, source)