docs: expand FAQ with placeholders and URLs as skin usage

docs/src/fancynpcs/faq.md

@@ -19,4 +19,26 @@ No, currently there is no support for ModelEngine 4 or any other model engine. S
 
 FancyNpcs does not have a movement system yet. This is a highly requested feature and might be added in the future. Keep an eye on the discord server for updates.
 
-We already made plans for a waypoint system, but it is not yet implemented. If you have any suggestions or ideas, feel free to share them on the discord server.
+We already made plans for a waypoint system, but it is not yet implemented. If you have any suggestions or ideas, feel free to share them on the discord server.
+
+### Placeholders as skin
+
+Using placeholders as skin does generally work. There are a few requirements tho:
+- The placeholder must resolve into a valid username, UUID, URL or file path
+- The placeholder must be able to resolve without any player context
+
+You can test if this is the case by trying to manually resolve the placeholder using the `/papi parse --null %YOUR-PLACEHOLDER%` command. 
+If the placeholder resolves to a valid value, you can use it as skin.
+
+### URLs as skin
+
+You can use URLs as skin. There are a few requirements tho:
+
+- The URL must be reachable
+- The URL must point to a valid skin file
+- Using a URL that points to a .png file is recommended
+
+How to use a skin from NameMC:
+
+![Click on the skin](../static/skin-url-namemc-1.png)
+![Right click on the download button](../static/skin-url-namemc-2.png)