CONTRIBUTING.md in h3-3.7.1

- old
+ new
@@ -7,10 +7,10 @@
 ## Pull requests
 
 * Please include tests that show the bug is fixed or feature works as intended.
 * Please add a description of your change to the Unreleased section of the [changelog](./CHANGELOG.md).
 * Please open issues to discuss large features or changes which would break compatibility, before submitting pull requests.
-* Please keep H3 compatible with major C compilers, such as GCC, Clang, and MSVC.
+* Please keep H3 compatible with major C compilers, such as GCC, Clang, and MSVC. We use clang-format-9 for source code formatting, if you have another version the CI job may error on formatting differences.
 * Please keep code coverage of the core H3 library at 100%.
 
 Before we can merge your changes, you must agree to the [Uber Contributor License Agreement](https://cla-assistant.io/uber/h3).
 
 ## Other ways to contribute