Add Darkfish deprecation warning

lib/rdoc/generator/darkfish.rb

@@ -212,6 +212,10 @@ def write_style_sheet
   # objects containing the extracted information.
 
   def generate
+    if self.class == RDoc::Generator::Darkfish
+      warn "The Darkfish template is deprecated and will be removed in version 9.0."
+    end
+
     setup
 
     write_style_sheet

test/rdoc/generator/aliki_test.rb

@@ -59,6 +59,14 @@ def teardown
     FileUtils.rm_rf @tmpdir
   end
 
+  def test_generate_does_not_warn_about_darkfish_deprecation
+    _, err = capture_output do
+      @g.generate
+    end
+
+    assert_empty err
+  end
+
   def test_inheritance_and_template_dir
     assert_kind_of RDoc::Generator::Darkfish, @g
     assert_match %r{/template/aliki\z}, @g.template_dir.to_s

test/rdoc/generator/darkfish_test.rb

@@ -69,6 +69,14 @@ def teardown
     FileUtils.rm_rf @tmpdir
   end
 
+  def test_generate_warns_about_deprecation
+    _, err = capture_output do
+      @g.generate
+    end
+
+    assert_equal "The Darkfish template is deprecated and will be removed in version 9.0.\n", err
+  end
+
   def test_generate
     top_level = @store.add_file 'file.rb'
     top_level.add_class @klass.class, @klass.name