describe how to debug for contributors (#17)

CONTRIBUTING.md

@@ -35,6 +35,35 @@ cargo fmt -- --check
 
 If you use [cargo-watch][], `cargo watch-check` alias is useful to run checks automatically on writing to a file.
 
+## Print debug
+
+Since this crate uses stdout, `println!` is not available for debugging. Instead, stderr through [`eprintln!`][eprintln]
+or [`dbg!`][dbg] are useful.
+
+At first, add prints where you want to debug:
+
+```rust
+eprintln!("some value is {:?}", some_value);
+dbg!(&some_value);
+```
+
+Then redirect stderr to some file:
+
+```sh
+cargo run -- --example minimal 2>debug.txt
+```
+
+Then the debug prints are output to the `debug.txt` file. If timing is important or you want to see the output in real-time,
+printing the file content with `tail` command would be useful.
+
+```sh
+# In a terminal, reproduce the issue
+cargo run -- --example minimal 2>debug.txt
+
+# In another terminal, run `tail` command to monitor the content
+tail -F debug.txt
+```
+
 ## Running a fuzzer
 
 To run fuzzing tests, [cargo-fuzz][] is necessary.
@@ -61,3 +90,5 @@ See [README in bench/](./bench/README.md) for more details.
 [cargo-watch]: https://crates.io/crates/cargo-watch
 [cargo-fuzz]: https://github.com/rust-fuzz/cargo-fuzz
 [criterion]: https://github.com/bheisler/criterion.rs
+[eprintln]: https://doc.rust-lang.org/std/macro.eprintln.html
+[dbg]: https://doc.rust-lang.org/std/macro.dbg.html