define pCodec

pCodec is undefuned in README.md
But it is defined in [example](https://github.com/leandromoreira/ffmpeg-libav-tutorial/blob/cc1d75b7e5f946b1cf4ed827531e521e625e70e9/0_hello_world.c#L135)

I think you should mark that _pCodec = pLocalCodec_ 
OR use
`AVCodecContext *pCodecContext = avcodec_alloc_context3(pLocalCodec);`
in README.md