docs(contributing): add link to Google Cloud Samples Style Guide
docs(contributing): add link to Google Cloud Samples Style Guide

This reference is being added to all -docs-samples repos in GoogleCloudPlatform.
GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples