docs: update CONTRIBUTING.md to include code formatting (#534)

Co-authored-by: Jeff Ching <chingor@google.com>
Co-authored-by: Jeffrey Rennie <rennie@google.com>

synthtool/gcp/templates/java_library/CONTRIBUTING.md

@@ -99,7 +99,16 @@ mvn -Penable-samples clean verify
     ```
 
 2. [Activate](#profile-activation) the profile.
-3. Define your samples in a normal Maven project in the `samples/` directory
+3. Define your samples in a normal Maven project in the `samples/` directory.
+
+### Code Formatting
+
+Code in this repo is formatted with
+[google-java-format](https://github.com/google/google-java-format).
+To run formatting on your project, you can run:
+```
+mvn com.coveo:fmt-maven-plugin:format
+```
 
 ### Profile Activation