Added missing UFL design guideline (#7429)

Author: colinloretz

docs/discord_social_sdk/design_guidelines/unified_friends_list.mdx

@@ -67,6 +67,16 @@ When referring to a player's Discord Identity, please use their **Discord Displa
 
 ![Discord display names](social-sdk/design-guidelines/UFL-07.png)
 
+## Scaling to Multiple Platforms
+
+If your game uses more than one platform's friend graph, combine them into "Online Elsewhere."
+
+Prioritize player needs by elevating available players to the top of the friends list, regardless of platform.
+
+Avoid friend-lists organized by company. This hierarchy dilutes the value-prop of putting online players first, in favor of platform delineation.
+
+![Scaling to Multiple Platforms](social-sdk/design-guidelines/UFL-08.png)
+
 ---
 
 ## Resources