Update CONTRIBUTING.md (GoogleCloudPlatform#3173)

Additional clarification on what to contribute.

Fixes #issue

> It's a good idea to open an issue first for discussion.

- [x] I have followed [Sample Format Guide](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/master/SAMPLE_FORMAT.md)
- [x] Appropriate changes to README are included in PR
- [x] Please **merge** this PR for me once it is approved.

Author: lesv

CONTRIBUTING.md

@@ -3,6 +3,13 @@
 We'd love to accept your patches and contributions to this project. There are
 just a few small guidelines you need to follow.
 
+In this repository, we are looking for patches that:
+* Improve readibility
+* Fix bugs
+* Improve clarity and understandability
+
+If you want to contribute a full sample / tutorial, please consider contributing to our [community pages](https://cloud.google.com/community) [[How To](https://cloud.google.com/community/tutorials/write)] ([code](https://github.com/GoogleCloudPlatform/community)).
+
 ## Contributor License Agreement
 
 Contributions to this project must be accompanied by a Contributor License