feat: add sas open source contrib handbook ref to contributing.md (VIYAARKCD-304)

Author: kevinlinglesas

CONTRIBUTING.md

@@ -17,3 +17,8 @@ All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose. Consult
 [GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
 information on using pull requests.
+
+## SAS Open Source Contributor Handbook
+
+Please review the [SAS Open Source Contributor Handbook](https://sassoftware.github.io/contributor-handbook.html)
+for additional information before contributing.