Add description of expect_num_outputs to planemo FAQ.

Author: jmchilton

docs/writing_how_do_i.rst

@@ -249,6 +249,18 @@ to the ``test`` element.
 
 - `job_properties.xml <https://github.com/galaxyproject/galaxy/blob/dev/test/functional/tools/job_properties.xml>`__
 
+------------------------------------------
+\.\.\. test output filters work?
+------------------------------------------
+
+If your tool contains ``filter`` elements, you can't verify properties of outputs
+that are filtered out and do not exist. The ``test`` element may contain an
+``expect_num_outputs`` attribute to specify the expected number of outputs, this
+can be used to verify that outputs not listed are expected to be filtered out during
+tool execution.
+
+- `output_filter.xml <https://github.com/galaxyproject/galaxy/blob/dev/test/functional/tools/output_filter.xml>`__
+
 ------------------------------------------
 \.\.\. test metadata?
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