add some info on frontendmode

frontend/frontendmode.txt

+example in python to parse ffmpeg2theora in frontend mode:
+
+'''
+import os
+
+cmd = "ffmpeg2theora  --frontend ..."
+f_stdout, f_stdin, f = os.popen3(cmd)
+info = {}
+line = f.readline()
+while line:
+    if line.startswith('f2t'):
+        for o in line.split(';')[1:]:
+            oo = o.split(': ')
+            if len(oo) >= 2:
+                info[oo[0]] = ": ".join(oo[1:]).strip()
+
+    #do something with info dict here
+
+    line = f.readline()
+'''
+