fix include error, declare AVCodec* as const, and update the example in README.md

[reidevries]

I'm not really sure why but it seems like Packet.hpp wasn't included in Decoder.hpp for some reason. Also AVCodec* needed to be declared as const, this might be due to version differences in libav, I am using a pretty recent version cloned from their github a few weeks ago. Feel free to ignore this if you want to stick with an older version i guess