Add PyPI migration guide for third-party services

This is a step towards pypi#2935.

docs/api-reference/index.rst

@@ -11,3 +11,4 @@ parts of Warehouse that generate them.
     json
     legacy
     xml-rpc
+    integration-guide

docs/api-reference/integration-guide.rst

@@ -0,0 +1,41 @@
+Integration guide
+=================
+
+We provide multiple APIs to help you integrate with PyPI; see
+:doc:`index`.
+
+
+Migrating to the new PyPI
+-------------------------
+
+If your site/service used to link or upload to pypi.python.org, you
+should start using pypi.org instead.
+
+Here are some tips.
+
+.. note::
+  ``{name}`` is the name of the package as represented in the URL;
+  for ``https://pypi.org/project/arrow/``, you'd insert ``arrow``
+  wherever you see ``{name}``.
+
+* If your client correctly follows redirects, you can replace
+  ``pypi.python.org`` in your links with ``pypi.org`` and everything
+  should just work. ``https://pypi.org/pypi/{name}`` (with or
+  without a trailing slash) redirects to
+  ``https://pypi.org/project/{name}/``.
+
+* In case you prefer a shorter URL: feel free to link to
+  ``https://pypi.org/p/{name}/``, which will redirect to
+  ``https://pypi.org/project/{name}/``.
+
+* JSON API: ``https://pypi.org/pypi/{name}/json`` returns the
+  expected JSON response directly -- see :doc:`json`.
+
+* Package upload RSS feed: ``https://pypi.org/pypi?%3Aaction=rss``
+  redirects to ``https://pypi.org/rss/updates.xml``. See
+  :doc:`feeds`.
+
+If you're a PyPI end user or packager looking to migrate to the new
+PyPI, please see `the official Python Packaging User Guide on
+migrating to PyPI
+<https://packaging.python.org/guides/migrating-to-pypi-org/>`_.