More user friendly guidance

[hamishwillee]

There are better docs for getting to understand MAVLink. Essentially they are less "technical" and do not attempt to cover everything.

• https://discuss.ardupilot.org/t/mavlink-step-by-step/9629
• http://api.ning.com/files/i*tFWQTF2R*7Mmw7hksAU-u9IABKNDO9apguOiSOCfvi2znk1tXhur0Bt00jTOldFvob-Sczg3*lDcgChG26QaHZpzEcISM5/MAVLINK_FOR_DUMMIESPart1_v.1.1.pdf

We should learn from these and restructure for target audiences - those wishing to use MAVLink, or to create their own generators.

[hamishwillee]

This should include info about system id allocation: #62