Document supported VPP versions

[chrismetz09]

We should document or clarify supported VPP versions.

ligato/vpp-agent#1812 (WIP)

ligato/vpp-agent#1808 (closed)

Seem to recall another conversation

Section on Supported VPP versions is needed. Contents might be:

• check vpp.env
• Browse changelog
• reminder of https://docs.ligato.io/en/latest/user-guide/agentctl/#vpp
• some notes like: If you want to build agent with different VPP version run make images VPP_VERSION=2009.

Please comment so I can add.

tx!

[VladoLavor]

Sounds good, afaik, supported versions are tracked in vpp.env only, a clear reference from docs might be useful.