Add versioning note to README

Author: jmoldow

README.rst

@@ -28,8 +28,34 @@ Installing
 
 .. code-block:: console
 
-    pip install boxsdk
-
+    pip install "boxsdk>=1.5,<2.0"
+
+Important Versioning Note
+-------------------------
+
+The production version of the ``boxsdk`` package is the 1.5.x release track.
+The code can be found on Github at
+<https://github.com/box/box-python-sdk/tree/1.5>. This is what will be
+installed with ``pip install boxsdk``.
+
+The development version of the ``boxsdk`` package is the 2.0.0 release track,
+currently in an alpha release.  The code can be found on Github at
+<https://github.com/box/box-python-sdk/tree/master>. This isn't installed by
+``pip`` by default, but can be installed if specifying a specific version. For
+example, ``pip install boxsdk==2.0.0a5``. We welcome early testing and feedback
+on these alpha releases. Note that while we're still in alpha, version 2.0.0 is
+subject to change, and there might be breaking changes between alpha releases
+(e.g. between 2.0.0a2 and 2.0.0a3).
+
+The 1.5.x release track will receive bugfix support at least until the final
+release of 2.0.0.
+
+We always recommend pinning to a specific version (e.g. by adding
+``boxsdk==1.5.3`` in your ``requirements.txt`` file), so that you don't risk
+accidentally upgrading to a new version and breaking your application. This is
+particularly important with the upcoming 2.0.0 release. There will be some
+breaking changes in version 2.0.0, which might break your application if you
+upgrade without adapting your code first.
 
 Authorization
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