add some clarification about when reducer loads the document again; d…

…ocument source_header_attributes [skip ci]

README.adoc

@@ -197,8 +197,10 @@ The reducer then uses a tree processor extension to fold the include stack into
 It does so by working from the end of the stack and inserting the lines into the parent until the stack has been flattened.
 As it goes, it also removes lines that have been excluded by the preprocessor conditionals as well as the directive lines themselves (unless the option to preserve conditionals has been specified).
 
-Finally, it loads the document again and returns it.
-The reduced source is available on the reconstructed document (via `Document#source` or `Document#source_lines`).
+If the sourcemap is enabled, it loads the document again.
+Finally, it returns the document.
+The reduced source is available on the reconstructed document via `Document#source` or `Document#source_lines`.
+The source header attributes (those defined in the header of the document) are available via `Document#source_header_attributes`.
 
 === Impact on Extensions