README: Add --input-service option.

README

@@ -36,6 +36,7 @@ Input options:
                             .    -I -              (read from stdin) (default)
  -R --input-rtp             | Enable RTP input
  -z --input-ignore-disc     | Do not report discontinuty errors in input.
+ -M --input-service <srvid> | Choose service id when input is MPTS.
 
 Output options:
  -O --output <dest>         | Where to send output. File or multicast address.