Contributing: add a note about code formatting

Wanted to share that pre-commit hook :)

CONTRIBUTING.md

@@ -11,3 +11,20 @@ Contributing entails that all contributions follow the [MPL 2.0 license](https:/
 
 Commit messages should be both _clear_ and _descriptive_. If possible, commit titles should start with a verb describing the change. Don't be shy to include additional information such as motivation for the change in the commit body. Be sure to ensure other developers will be able to understand _why_ a specific change has occurred in the future.
 
+## Code formatting
+
+To ensure consistent formatting, use `clang-format -i **/*.[ch]` (beware, not all shells support that glob). Don't worry too much about forgetting to do so, the pipeline will remind you by failing on your commit/PR. You can also make this your pre-commit git hook (see `man 5 githooks`) to avoid committing any misformatted code:
+
+```sh
+#!/bin/sh
+
+# Check if there're any formatting errors
+find . '(' -name .git -o -path ./build ')' -prune \
+    -o -type f -name '*.[ch]' -print |
+  xargs -d '\n' clang-format --dry-run >/dev/null 2>&1
+
+if [ $? -ne 0 ]; then
+  echo 'Code formatting is wrong, fix it before commit' >/dev/stderr
+  exit 1
+fi
+```