Add language around Cloudflare IPs

See https://about.gitlab.com/blog/2020/01/16/gitlab-changes-to-cloudflare/ for details.

doc/user/gitlab_com/index.md

@@ -96,7 +96,9 @@ GitLab.com is using the IP range `34.74.90.64/28` for traffic from its Web/API
 fleet. You can expect connections from webhooks or repository mirroring to come
 from those IPs and whitelist them.
 
-For connections from CI/CD runners we are not providing static IP addresses.
+GitLab.com is fronted by Cloudflare. For incoming connections to GitLab.com you might need to whitelist CIDR blocks of Cloudflare ([IPv4](https://www.cloudflare.com/ips-v4) and [IPv6](https://www.cloudflare.com/ips-v6))
+
+For outgoing connections from CI/CD runners we are not providing static IP addresses.
 All our runners are deployed into Google Cloud Platform (GCP) - any IP based
 firewall can be configured by looking up all
 [IP address ranges or CIDR blocks for GCP](https://cloud.google.com/compute/docs/faq#where_can_i_find_product_name_short_ip_ranges).