Add documentation for configuration of Fastly-Client-IP header

After reading some comments and issues it should be documented some points:

1. Configure the reverse proxy
   

     $settings['reverse_proxy'] = TRUE;
   

2. Configure the reverse proxy address, the problem is that the IP address from fastly are dynamic, so it should be calculate dynamically https://www.fastly.com/documentation/reference/api/utils/public-ip-list/
3. As the default header X-Forwarded-For is not secure, it should be used the header Fastly-Client-IP https://www.fastly.com/documentation/reference/http/http-headers/Fastly-... and configure the vlc
   

   if (fastly.ff.visits_this_service == 0 && req.restarts == 0) {
     set req.http.Fastly-Client-IP = client.ip;
   }
   

Added documentation on https://git.drupalcode.org/project/fastly/-/merge_requests/28/diffs#diff... issue ✨ Caclculate fastly ips dynamically Active