Document hb-subset

[rsheeter]

https://github.com/harfbuzz/harfbuzz/blob/main/docs/repacker.md is lovely, write up how the rest of subsetting works with code refs

[behdad]

• serializer design: https://goo.gl/bHvnTn
• subsetter design: https://goo.gl/Qy3Eqc

[rsheeter]

Do you need/want to add anything to that or can I close this?

[rsheeter]

I tend to think it still makes sense to describe how it works akin to how repacker.md does?

[behdad]

I feel like that can be a task for Garret now.

[rsheeter]

🚌=>@garretrieger

[garretrieger]

I just added a short writeup of how the serializer works in hb here: https://github.com/harfbuzz/harfbuzz/blob/main/docs/serializer.md

[rsheeter]

I think we can call this complete, thank you. I'll add links to the root readme.