Prefer small PRs

Author: Philippe-Cholet

CONTRIBUTING.md

@@ -9,6 +9,8 @@ then **please first file an issue** describing your proposal._
 
 To pass CI tests successfully, your code must be free of "compiler warnings" and "clippy warnings" and be "rustfmt" formatted.
 
+Note that small PRs are easier to review and therefore are more easily merged.
+
 ## Write a new method/adaptor for `Itertools` trait
 In general, the code logic should be tested with [quickcheck](https://crates.io/crates/quickcheck) tests in `tests/quick.rs`
 which allow us to test properties about the code with randomly generated inputs.