chore: add style guide to CONTRIBUTING.md (GoogleCloudPlatform#1677)

Author: bshaffer

CONTRIBUTING.md

@@ -31,6 +31,10 @@ accept your pull requests.
 1. Ensure that your code has an appropriate set of unit tests which all pass.
 1. Submit a pull request.
 
+## Writing a new sample
+
+Write samples according to the [sample style guide](https://googlecloudplatform.github.io/samples-style-guide/).
+
 ## Testing your code changes.
 
 ### Install dependencies