added requirement to use checkstyle in contributing file

Author: Cameron

CONTRIBUTING.md

@@ -10,6 +10,8 @@ If you want to contribute code:
 
 3. Pull requests are gladly accepted. If there are any changes that developers should be aware of, please update the [change log](CHANGELOG.md)
 
+4. Mapbox uses checkstyle to enforce good Java code standards, Make sure to read the [wiki entry](https://github.com/mapbox/mapbox-java/wiki/Setting-up-Mapbox-checkstyle) and setup. CI will fail if your PR contains any mistakes. 
+
 # Code of conduct
 Everyone is invited to participate in Mapbox’s open source projects and public discussions: we want to create a welcoming and friendly environment. Harassment of participants or other unethical and unprofessional behavior will not be tolerated in our spaces. The [Contributor Covenant](http://contributor-covenant.org) applies to all projects under the Mapbox organization and we ask that you please read [the full text](http://contributor-covenant.org/version/1/2/0/).