If you've been lucky enough to write some software worth publishing, and then super-lucky enough to have others like it enough to use it; and then ultra-mega lucky enough to have someone to like it enough to send you a patch for it; then you may be lucky enough to need to have a credits file.
When people contribute to your projects, it's only polite to make a little note so that they can see that their contribution was appreciated. Some of my projects even credit my employer when I've been fortunate enough to be allowed to write some F/LOSS at work.
The same applies if you "borrow" code from another project. Be sure to comply with their licensing terms, and even if they don't require it, it's only polite to thank them for their work. So for your homework, I'd like you to go back to any software you've written which has received patches from others and be sure to credit your contributors clearly and fully.