The Creative Commons website gives this example for giving attribution to an image that was found on a website. For this project, contributors who wish to contribute assets would be sending the assets to me directly/through git and would likely not be hosted anywhere else.
In your README, include a statement for each asset used,
“Foo” by bar is licensed under CC BY 2.0
"Baz" by qux is licensed under CC BY 2.0
Note: bar and qux would be the names of the original creator of each asset specifically.
Or, you can create a separate file, e.g.,
assets/NOTICE; and include them there.