diff --git a/README.md b/README.md
index 9d9e85e..33f7907 100644
--- a/README.md
+++ b/README.md
@@ -168,3 +168,7 @@ ford ford.yml
 
 Contributions to `ForImage` are welcome!
 If you find any issues or would like to suggest improvements, please open an issue.
+
+## References
+
+- https://en.wikipedia.org/wiki/Netpbm#File_formats
\ No newline at end of file