Clarify what needs to happen for MAVLink Routing

[hamishwillee]

A few notes to add for routing:

• A MAVLink IP-based router does not need to check the CRC because these protocols have their own packet CRC.
• Unfortunately CRC_EXTRA prevents you from routing an unknown packet.
• The end user (not router) still needs to check the CRC, in order to get CRC_EXTRA and verify that the message is compatible.

@LorenzMeier -From mavlink email group:

• Is it possible to disable the CRC_EXTRA check so that routers can parse messages without having to know whether or not they are supported by the end point.
• If not, propose we add a compile-time flag to disable CRC checks. OK?

[hamishwillee]

See also https://github.com/mavlink/mavlink-devguide/pull/113