As specified by the COPYING file, fio is free software published under version
2 of the GPL license. That covers the copying part of the license. When using
fio, you are encouraged to uphold the following moral obligations:

- If you publish results that are done using fio, it should be clearly stated
  that fio was used. The specific version should also be listed.

- If you develop features or bug fixes for fio, they should be sent upstream
  for inclusion into the main repository. This isn't specific to fio, that
  is a general rule for any open source project. It's just the Right Thing
  to do. Plus it means that you don't have to maintain the feature or change
  internally. In the long run, this is saving you a lot of time.

I would consider the above to fall under "common courtesy", but since
people tend to have differing opinions of that, it doesn't hurt to spell out
my expectations clearly.