Add note about forcing timestamps
Add note about forcing timestamps

This is useful if you're trying to minimize the differences between
generated .fw files, and it hadn't been documented.

michaelkschmidt/fwup,michaelkschmidt/fwup,fhunleth/fwup,fhunleth/fwup,michaelkschmidt/fwup,fhunleth/fwup