Incorporate patch review on Flake8Command

* add a note about the setuptools command to README
* don't install the Flake8Command entry point without setuptools
* fix a bug in Flake8Command that might cause double checking
  of files in 'packages'

README

@@ -123,6 +123,31 @@ In order to use Flake8 inside a buildout, edit your buildout.cfg and add this::
     entry-points =
         flake8=flake8.run:main
 
+
+setuptools integration
+======================
+
+If setuptools is available, Flake8 provides a command that checks the
+Python files declared by your project. To use it, add flake8 to your
+setup_requires::
+
+    setup(
+        name="project",
+        packages=["project"],
+
+        setup_requires=[
+            "flake8"
+        ]
+    )
+
+Running ``python setup.py flake8`` on the command line will check the
+files listed in your ``py_modules`` and ``packages``. If any warnings
+are found, the command will exit with an error code::
+
+    $ python setup.py flake8
+    
+
+
 Original projects
 =================