Shield overall description

Add or extend overall Shield description to make clear what it exactly is.

Shield

• Is not a MOX
• Is not modular/extensible
• Has automatic updates enabled and not configurable
• Has Turris Sentinel, OpenVPN and NetMetr preinstalled
• Does not have full-value Turris OS
  • Does not have LuCI
  • Does not include options to install extra software
  • ReForis interface is pruned and crippled (e.g. no Guest network)
• The only supported setup is a router with WAN Ethernet uplink (PPPoE possible) and 4x LAN Ethernet downlink, with IPv4 NAT and IPv6 prefix delegation (from DHCPv6)

Also, we must mark visibly some software features that are not present or supported on Shield.