add describe to gab rewrite

lib/pleroma/web/activity_pub/mrf/gab_rewrite_policy.ex

@@ -36,6 +36,9 @@ defmodule Pleroma.Web.ActivityPub.MRF.GABRewritePolicy do
     put_in(object, ["object", "content"], content <> " " <> Enum.random(@appends))
   end
 
+  @impl true
+  def describe, do: "Appends boomer messages to the end of all statuses coming from gab.com"
+
   @impl true
   def filter(%{"type" => "Create", "actor" => actor} = object) do
     actor_info = URI.parse(actor)