Update CONTRIBUTING.md with a link to the Contributing page on about.gitlab.com

CONTRIBUTING.md

@@ -50,6 +50,9 @@ _This notice should stay as the first item in the CONTRIBUTING.md file._
 
 ## Contribute to GitLab
 
+For a first-time step-by-step guide to the contribution process, see
+["Contributing to GitLab"](https://about.gitlab.com/contributing/).
+
 Thank you for your interest in contributing to GitLab. This guide details how
 to contribute to GitLab in a way that is efficient for everyone.