Caddy: Add note about redirect requirement for ActivityPub

no ref

- Our ActivityPub framework lets users set their AP username to index@example.com and have Ghost on www.example.com (ie instead of having their AP user be index@www.example.com)
- To do this users _must_ redirect from the non-www domain -> the www domain as the AP protocol will do lookups on the non-www path to validate and communicate to that username on the web
- Without this AP will fail since domains can't communicate with AP as there wont be redirects for them to follow

caddy/Caddyfile.example

@@ -51,6 +51,7 @@
 
 # If you want to redirect the non-www variant of your domain to the www varient uncomment the below 4 lines and replace REPLACE_ME with your domain
 # Note: You must have DNS setup correctly for this to work
+# If you're want to enable ActivityPub you _must_ enable this redirect
 # REPLACE_ME {
 # 	import snippets/Logging
 # 	redir https://REPLACE_ME{uri}