Doc on signing?

[hamishwillee]

Hi @WickedShell ,

Do you have any info on how to implement signing on this Clojure library? I'm updating the message signing on mavlink.io and I'd like to link to it.

The info I'd hope to see would cover:

1. How to enable signing for the library (ie turn it on for a particular set of message)
2. What variable is used to set/access the signing bit in the incompatibility flags
3. What API is used for determining what unsigned messages are accepted
4. Where the secret has to be stored/accessed by code.