Message Definitions need to include how to define messages

[hamishwillee]

We need information about how to create new messages, including

• Style guide (e.g. for punctuation). Not including units in the text (except if conditional on something, so multiple options possible). Naming params etc. I wonder if we can astyle this?
• Units - what they are.
• General comments about message format/schema, and in particular about the allowed units: https://github.com/ArduPilot/pymavlink/blob/master/generator/mavschema.xsd#L58

This might be a new document, or as part of Message Definitions - but either way should be linked from message definitions.

We might also try auto-import the XML via gitbook.

Discussion on this started here: mavlink/mavlink#914 (comment)

[amilcarlucas]

PLEASE: avoid "conditional on something" it generates a documentation mess and using the generated code is also stranger.

[hamishwillee]

Duplicate #67