Add notes to sites.conf

src/whitelist/sites.conf

@@ -1,10 +1,14 @@
 #
-# List of whitelisted services
+# List of whitelisted endpoints that allowed for server communication.
+# For users with free plan, the whitelist is applied, we call them "Firewall"
+# Anything outside this list will be blocked, unless the firewall disabled.
+# This is made to prevent our servers from participating in DDoS attack.
+# You're welcome to add more public endpoint here, as long as it's:
+# 1. Not behind a paywall (Free access with API token is fine)
+# 2. Has public documentation (Please mention the doc when submitting PR)
+#
+# Read more https://domcloud.co/docs/features/security#outbound-connection
 #
-# It must be:
-# 1. Completely free service
-# 2. Used by a lot of people
-# 3. Easily handle million hits
 #
 # GitHub
 github.com