docs(fronting-groups): add redd.it to the fastly group's domains list

Reddit serves images from redd.it (their CDN-style image host) which is
also on Fastly. Without this entry, the example config matches reddit.com
but image loads still fall back to direct, which is unreliable from Iran
ISPs. Suggested by @Shjpr9 in #502.

Author: therealaleph

config.fronting-groups.example.json

@@ -32,6 +32,7 @@
         "reddit.com",
         "redditstatic.com",
         "redditmedia.com",
+        "redd.it",
         "githubassets.com",
         "githubusercontent.com",
         "pypi.org",