Hosting projects on Github: ban on storing font binaries

This topic comes up every year in discussions so it is worthy of an issue. It can be added to https://googlefonts.github.io/gf-guide/hosting#commits-manage-progress-and-track-changes

Several of us believe we shouldn't be storing font binaries in the repo for the following reasons:

- The binaries may have been built from earlier commits or not the correct build chain
- They take up a lot of space
- They're not diffable.

Font binaries should be stored in tagged releases instead.