added docs for DomainFilter usage

lib/crawly/middlewares/domain_filter.ex

@@ -1,12 +1,23 @@
 defmodule Crawly.Middlewares.DomainFilter do
   @moduledoc """
-  Filters out requests which are going outside of the crawled domain
+  Filters out requests which are going outside of the crawled domain.
+
+  The domain that is used to compare against the request url is obtained from the spider's `c:Crawly.Spider.base_url` callback.
+
+  Does not accept any options. Tuple-based configuration optionswill be ignored.
+
+  ### Example Declaration
+  ```
+  middlewares: [
+    Crawly.Middlewares.DomainFilter
+  ]
+  ```
   """
 
   @behaviour Crawly.Pipeline
   require Logger
 
-  def run(request, state) do
+  def run(request, state, _opts \\ []) do
     base_url = state.spider_name.base_url()
 
     case String.contains?(request.url, base_url) do