Improve spec by including info on encoding and layer.json

[b-davis77]

For devs who want to create their own quantized mesh tiles, it would be helpful in the spec to have algorithms describing that process. It's great that there are very specific algorithms on the decoding process, but that makes the assumption that there is data already encoded. Some applications may need to create their own tile sets dynamically and serve them.

The encoding process has been the biggest hurdle for me, and I imagine others have faced similar issues.

Also, I've found various information scattered about regarding the layer.json file. Much more information on this file would be extremely helpful.