Adding 'Upgrading to 2.x' doc.

docs/source/index.rst

@@ -14,6 +14,7 @@ Python Myfitnesspal
  how_to/index
  contributing
  api/index
+ upgrading
 
 Do you track your eating habits on `MyFitnessPal <https://www.myfitnesspal.com/>`_? Have you ever wanted to analyze the information you're entering into MyFitnessPal programatically? 
 

docs/source/upgrading.rst

@@ -0,0 +1,50 @@
+Upgrading from 1.x to 2.x
+=========================
+
+Between the 1.x and 2.x versions of this library,
+the mechanism used for authentication changed,
+and this change has an impact
+on how you instantiate the ``myfitnesspal.Client`` object.
+
+For more information about why this change was necessary,
+see `Issue #144 <https://github.com/coddingtonbear/python-myfitnesspal/issues/144>`_.
+
+Version 1.x **(Obsolete)**
+--------------------------
+
+Before getting started,
+you would store a password in your system keyring
+for the user account you would like to use
+(in this example: 'myusername').
+
+In your code, you would then instantiate your
+``myfitnespal.Client`` like this:
+
+.. code:: python
+
+ import myfitnesspal
+
+ client = myfitnesspal.Client('myusername')
+
+Version 2.x **(Current)**
+-------------------------
+
+Before getting started,
+now you should open a web browser on the same computer
+you will be using this library from,
+go to `https://myfitnesspal.com/ <https://myfitnesspal.com/>`_,
+and log in to MyFitnessPal using
+the user account you would like to use.
+
+In your code, you can then instantiate your
+``myfitnespal.Client`` like this:
+
+.. code:: python
+
+ import myfitnesspal
+
+ client = myfitnesspal.Client()
+
+Note that the instantiation no longer accepts a username,
+and instead reads the log in information directly
+from your browser.