Add docs

docs/setuptools.rst

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+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 warning
+is found, the command will exit with an error code::
+
+    $ python setup.py flake8