Added note to test docstring about the minimum capacity of filesystem required

fs/tests/__init__.py

@@ -40,6 +40,8 @@ class FSTestCases(object):
     as a mixin for your own unittest.TestCase subclass and have the setUp
     method set self.fs to an instance of your FS implementation.
     
+    NB. The Filesystem being tested must have a capacity of at least 3MB.
+
     This class is designed as a mixin so that it's not detected by test
     loading tools such as nose.
     """