document default filter

lib/liquid/standardfilters.rb

@@ -421,6 +421,16 @@ module Liquid
       result.is_a?(BigDecimal) ? result.to_f : result
     end
 
+    # Set a default value when the input is nil, false or empty
+    #
+    # Example:
+    #    {{ product.title | default: "No Title" }}
+    #
+    # Use `allow_false` when an input should only be tested against nil or empty and not false.
+    #
+    # Example:
+    #    {{ product.title | default: "No Title", allow_false: true }}
+    #
     def default(input, default_value = '', options = {})
       options = {} unless options.is_a?(Hash)
       false_check = options['allow_false'] ? input.nil? : !input