Add "Testing" section to the "CONTRIBUTING.md" file

CONTRIBUTING.md

@@ -22,6 +22,10 @@ Pull requests represent a proposed solution to a specified problem.  They should
 
 For more on how 10up writes and manages code, check out our [10up Engineering Best Practices](https://10up.github.io/Engineering-Best-Practices/).
 
+### Testing
+
+Helping to test an open source project and provide feedback on success or failure of those tests is also a helpful contribution.  You can find details on the Critical Flows and Test Cases in the respective GitHub Wiki as well as details on our overall approach to [Critical Flows and Test Cases in our Open Source Best Practices](https://10up.github.io/Open-Source-Best-Practices/testing/#critial-flows).  Submitting the results of testing via our Critical Flows as a comment on a Pull Request of a specific feature or as an Issue when testing the entire project is the best approach for providing testing results.
+
 ## Workflow
 
 The `develop` branch is the development branch which means it contains the next version to be released.  `trunk` contains the latest released version as reflected in the WordPress.org plugin repository.  Always work on the `develop` branch and open up PRs against `develop`.