Add info on how to customize mavlink

[hamishwillee]

This is something you might want to do in order to test new messages being added to a flight stack, or if you're customizing QGC for another flight stack and you want to update the whole dialect. I figured providing some hints might be useful.

@DonLakeFlyer Is this information:

• correct
• sufficient
• in the right place?

Is there a better way to do this?

[DonLakeFlyer]

Yup, this is what you do. The key thing is that QGC uses the ardupilot dialect.

[DonLakeFlyer]

"Replace the pre-build C library a" I've only done this once myself ages ago but I seem to remember something quirky with this step. Something to do with building the custom C Library and then moving it over the QGC. Only vaguely remember having problems with it. Have tried doing it yourself end to end? Maybe it's all good now.

[hamishwillee]

Thanks @DonLakeFlyer I haven't tried it end to end, because I'm still waiting for the docker build to work properly.

But I'm happy to merge as-is and have people report bugs against this.