Detect generated files on Github

[mimoo]

Github seems to use this code to detect generated files: https://github.com/github/linguist/blob/master/lib/linguist/generated.rb

It'd be good to modify it to detect such files in OCaml (we can make the first line of these files (* file generated using ... *)). This way we can quickly skim them over in PRs and such.

In addition, it'd be good to ensure in CI that generated files were not over-written. I'm not sure what the list of generated files is but we have promoted:

• scalars.ml in pickles
• kimchi.ml in kimchi bindings