Track clarification in UbuntuSeededSnaps Policy

[Dimitri John Ledkov]

Currently https://wiki.ubuntu.com/UbuntuSeededSnaps does not specify
anything about tracks.

Currently the status quo is to seed the default-track of the snap, aka "latest".

Recently we have been approached by LXD upstream to use a different
"LTS" track of the seeded lxd snap which has longer support timeframes
and closer matches the timeframe of the Ubuntu Branch the seeded snap
tracks.

I would like to request additions to "Channel availability" section
something along the lines of

"The default track is used for seeding, which usually is called
latest. Publishers may request seeding from a different track, if it
better matches longevity of a given Ubuntu Series. Example: request to
use lts track for Ubuntu 20.04 LTS release."

-- 
Regards,

Dimitri.