Show a deprecation warning when '-e haml' or '-e slim' is specified

kaminari-core/lib/generators/kaminari/views_generator.rb

@@ -66,7 +66,14 @@ BANNER
       end
 
       def template_engine
-        options[:template_engine].try(:to_s).try(:downcase) || 'erb'
+        engine = options[:template_engine].try(:to_s).try(:downcase)
+
+        if engine == 'haml' || engine == 'slim'
+          ActiveSupport::Deprecation.warn 'The -e option is deprecated and will be removed in the near future. Please use the html2slim gem or the html2haml gem ' \
+                                          'to convert erb templates manually.'
+        end
+
+        engine || 'erb'
       end
     end