Add contributing_url

This means any repos which redirect here will pick up the standard
openfaas/faas guide instead of pointing to their local variant.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

.DEREK.yml

@@ -17,3 +17,5 @@ maintainers:
 features:
  - dco_check
  - comments
+
+contributing_url: https://github.com/openfaas/faas/blob/master/CONTRIBUTING.md