DOC: Mention eval routines

doc/writing_a_test.md

@@ -264,6 +264,10 @@ sp.call('flirt -in {} -ref {} -out {} -omat {}'.format(
     inimg, refimg, outimg, outxform).split())
 ```
 
+If you want to perform your own evaluation within your `feedsRun` script, you
+may find some of the routines in the `pyfeeds.evaluate` module useful. For
+example, the `evalImage` routine can be used to compare two NIfTI images.
+
 
 ## Other examples