diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 1bf3f44..2a89115 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -6,6 +6,11 @@ Feel free to open an issue anytime you wish to ask a question, suggest a feature
 
 1. Be nice to other people.
 2. If editing the Rust source code, remember to run `rustfmt` (otherwise, CI will warn you the code was not properly formatted).
+3. If new formats are added, please add the format to `tests/integration.rs`.
+If it is an archive format that handles directories, it should be added to `DirectoryExtension`, otherwise it should be added to `FileExtension`.
+It should be added to `mime.rs` as well if the [`infer`](https://docs.rs/infer) crate supports it.
+Most tests are written with `proptest` ([book](https://altsysrq.github.io/proptest-book/), [docs](https://docs.rs/proptest)).
+If you wish to improve these tests, the proptest book might help you.
 
 Note: we are using `unstable` features of `rustfmt`! Nightly toolchain is required (will likely be installed automatically, cause the toolchain was specified in the project root).