METADATA.md file noticably out of date

[n8willis]

The METADATA.md file in this repository describes the old JSON file, not the PB file currently in use.

I have an outside use case for storing metadata external to font binaries and would be interested in developing a solution that's aligned with, or at least compatible with, what Google Fonts is doing.

The existing protobuf docs I've seen all suggest that *.pb files are binary representations, so adding something here that describes what Google Fonts is doing in practice would be most helpful indeed.