add a warning header comment indicating that files are generated

[petergardfjall]

First off, thanks for a very useful tool!

For many code generators it is customary to add a header comment that indicates that the file is a generated one and, hence, shouldn't be edited. This can be very valuable information in a large code base.

For example, the following header could be added to every generated file.

// Code generated by quicktype; DO NOT EDIT.

If adding it as default behavior feels too disruptive, maybe it would be possible to opt-in on it with a --add-generated-warning command-line flag or similar?