Standardize comment warning about machine-generated files

[feep]

Qri machine-generates a lot of code, docs...

The heavily-bikeshedded go standard looks good to me.

A standard comment format would be easily greppable and cooperate with existing code tooling like linguist.

Generated files are marked by a line of text that matches the regular expression, in Go syntax:
^// Code generated .* DO NOT EDIT\.$
The .* means the tool can put whatever folderol it wants in there, but the comment must be a single line and must start with Code generated and end with DO NOT EDIT., with a period.

The text may appear anywhere in the file.

[feep]

This comment should be followed by additional comments specifying:

• URL of template
• URL of generation script
• Regeneration method: Automation URL, or how to manually generate