Update CONTRIBUTING.md
Update CONTRIBUTING.md

[Fix] dead links for 3.1
helpfulrobot/i-lateral-silverstripe-siteconfig,jonom/silverstripe-cms,silverstripe/silverstripe-siteconfig,silverstripe/silverstripe-cms,silverstripe-labs/silverstripe-reports,dhensby/silverstripe-cms,silverstripe-labs/silverstripe-reports,dhensby/silverstripe-reports,jonom/silverstripe-cms,dhensby/silverstripe-cms,silverstripe/silverstripe-cms,dhensby/silverstripe-reports,dhensby/silverstripe-cms