Add --allow-unsafe to update instructions

[richafrank]

Prior to this change, running pip-compile with --allow-unsafe affected the output file, in that setuptools et al would become uncommented. However, the output file's instructions on what command to re-run to produce it did not include --allow-unsafe, so setuptools would end up commented out again. See #664

This PR includes that flag in the instructions in the output file's header.

Changelog-friendly one-liner: Add --allow-unsafe to update instructions

cc @vphilippon as a reviewer

Contributor checklist

• Provided the tests for the changes
• Requested (or received) a review from another contributor
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md afterwards).

[atugushev]

Nice work! Have a couple of minor comments though.

[atugushev]

@richafrank thanks!

[richafrank]

Great - thanks @atugushev !