DOC: Add explanation to test plugin.

scikits/image/io/_plugins/test_plugin.py

@@ -1,3 +1,6 @@
+# This mock-up is called by ../tests/test_plugin.py
+# to verify the behaviour of the plugin infrastructure
+
 def imread(fname, as_grey=False, dtype=None):
     assert fname == 'test.png'
     assert dtype == 'i4'

scikits/image/io/tests/test_plugin.py

@@ -8,7 +8,7 @@ from copy import deepcopy
 
 def setup_module(self):
     self.backup_plugin_store = deepcopy(plugin.plugin_store)
-    plugin.use('test')
+    plugin.use('test') # see ../_plugins/test_plugin.py
 
 def teardown_module(self):
     plugin.plugin_store = self.backup_plugin_store