VIP20: Varnish ABI and packaging

[Dridi Boukelmoune]

Hello everyone,

As requested last week, I started a VIP regarding VMODs, Varnish ABI,
and packaging.

While this is still incomplete, please let me know if anything needs a
clarification so far. The research took me a while so I'll continue
later, probably next week.

https://github.com/varnishcache/varnish-cache/wiki/VIP20%3A-Varnish-ABI-and-packaging

Cheers,
Dridi