Update CONTRIBUTING.md (elastic#5916)

add top level link to Docs Guidelines docs.

CONTRIBUTING.md

@@ -11,4 +11,5 @@ This page is intended for contributors to the [Package Registry](https://github.
 * [Developer workflow: bug fix an old package version](./docs/developer_workflow_bug_fix_older_package_version.md) - step-by-step guide on how to release a new fix for an old package version.
 * [Fine-tune integration](./docs/fine_tune_integration.md) - fill missing items, correct structure, review manifests
 * [Testing and validation](./docs/testing_and_validation.md) - run the Elastic stack, use test runners, review test coverage
-* [Tips for building integrations](./docs/tips_for_building_integrations.md) - see your local changes in Kibana, follow best practices
+* [Tips for building integrations](./docs/tips_for_building_integrations.md) - see your local changes in Kibana, follow best practices
+* [Documentation Guidelines](./docs/documentation_guidelines.md) - guidelines for contributing to the docs, the Generic guidelines also has lots of docs tips