Add FAQ about how to tell what messages are supported

[hamishwillee]

This might not be the absolute best place for information, and you might argue that I'd be better stopping with just the following

Each system is expected to document its own support for MAVLink microservices, messages and commands.

BUT, I wanted it to be useful enough that I could refer people here to find out how to find out more.

[hamishwillee]

Fixes #110

[LorenzMeier]

@hamishwillee Could you please rebase? Thanks!

[hamishwillee]

@LorenzMeier rebased!