document function outputs

Author: James Halliday

readme.markdown

@@ -193,7 +193,8 @@ The output format for `fr` and each of the `fr` sub-streams given by each
 
 `opts.o` or `opts.outputs` should be an array that pairs up with the `files` array to specify
 where each bundle output for each entry file should be written. The elements in
-`opts.o` can be string filenames or writable streams.
+`opts.o` can be string filenames, writable streams, or functions that return a
+string filename or writable stream.
 
 `opts.entries` or `opts.e` should be the array of entry files to create
 a page-specific bundle for each file. If you don't pass in an `opts.entries`,