Doc: Upgrading to 3.0

[Tollef Fog Heen]

]] Andreas Plesner Jacobsen 

| Based on questions in the IRC channel, I've compiled a small list of changes to
| make it easier to upgrade from 2.1 to 3.0 with the hope that they can be
| included in the docs. rst is attached.

Thanks a lot, I've glued this into the docs now after adding
hit_for_pass and fixing up the markup slightly.

https://www.varnish-cache.org/docs/trunk/installation/upgrade.html for
the online version; I'll merge it to 3.0 later.

Best regards,
-- 
Tollef Fog Heen
Varnish Software
t: +47 21 98 92 64