Add a note to the sample config about the MRF policy customisations

priv/templates/sample_config.eex

@@ -82,6 +82,12 @@ config :joken, default_signer: "<%= jwt_secret %>"
 
 config :pleroma, configurable_from_database: <%= db_configurable? %>
 
+# When customising MRF policies, don't forget to include ObjectAgePolicy if you
+# don't want your TWKN to be flooded with posts from ages ago.
+#
+# config :pleroma, :mrf,
+#   policies: []
+
 <%= if Kernel.length(upload_filters) > 0 do
 "config :pleroma, Pleroma.Upload, filters: #{inspect(upload_filters)}"
 end %>