Add a contribution note about rspec

Improving our rspecs is another great place for contributors.

doc/development/contributing/index.md

@@ -86,6 +86,11 @@ If you would like to contribute to GitLab:
 - Issues with the
   [`~Accepting merge requests` label](issue_workflow.md#label-for-community-contributors)
   are a great place to start.
+- Optimizing our tests is another great opportunity to contribute. You can use
+  [RSpec profiling statistics](https://gitlab-org.gitlab.io/rspec_profiling_stats/) to identify
+  slowest tests. These tests are good candidates for improving and checking if any of
+  [best practices](../testing_guide/best_practices.md)
+  could speed them up.
 - Consult the [Contribution Flow](#contribution-flow) section to learn the process.
 
 If you have any questions or need help visit [Getting Help](https://about.gitlab.com/get-help/) to