Add style section to CONTRIBUTING.md.
Add style section to CONTRIBUTING.md.

I'd like to point
https://github.com/GoogleCloudPlatform/Template/wiki/style.html back to
the repositories where the samples should go.

GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples