Add instructions to bundle alac

[outsharded]

Added note on alac decoder, and instructions to add the decoder

[mikebrady]

Thanks for the suggestion. However, there are a couple of comments that I'd like to make about it.

1. Shairport Sync already has an ALAC decoder built in, so the Apple ALAC decoder is an option. It's not actually needed -- as your text would seem to imply -- for Shairport Sync to be able to decode ALAC-encoded material.

2. Experimentally, it was worth adding the Apple ALAC decoder as an option:

   1. It was not known whether the Apple decoder would decode material better than the built-in decoder, written by David Hammerton.
   2. The Apple decoder appears to be capable of handling different numbers of channels, bit depths and rates.

   As it turns out the Hammerton decoder has not, to my knowledge, been found to decode incorrectly and those extra formats are not used in AirPlay 2 or AirPlay.

3. The Apple decoder doesn't seem to be noticeably more or less efficient than the Hammerton decoder.

4. The Apple ALAC decoder is known to have security issues, and it doesn't seem to be maintained upstream any more. David Hammerton has provided the ALAC decoder in the FFmpeg decoder collection. Since FFmpeg is maintained, there is a better chance that security and other issues will be fixed there over time. In fact, I'd be hoping to add the FFmpeg ALAC decoder as a configuration option.

So it seems to me that we should not be drawing attention to the Apple ALAC decoder at this time. In fact, if anything, we should be de-emphasising or even deprecating it in the near future.