Merge pull request matplotlib#3349 from tacaswell/api_whats_new_folders

DOC : added folders for api_changes and whats_new

Author: efiring

doc/api/api_changes/README.rst

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+For changes which require an entry in `api_changes.rst` please create
+a file in this folder with the name :file:`YYYY-MM-DD-[initials].rst`
+(ex :file:`2014-07-31-TAC.rst`) with contents following the form: ::
+
+    Brief description of change
+    ```````````````````````````
+
+    Long description of change, justification, and work-arounds to
+    maintain old behavior (if any).

doc/users/whats_new/README.rst

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+This folder is for placing new portions of `whats_new.rst`.
+
+When adding an entry please look at the currently existing files to
+see if you can extend any of them.  If you create a file, name it
+something like :file:`cool_new_feature.rst` if you have added a brand new
+feature or something like :file:`updated_feature.rst` for extensions of
+existing features.  Include contents of the form: ::
+
+    Section Title for Feature
+    -------------------------
+
+    A bunch of text about how awesome the new feature is and examples of how
+    to use it.
+
+    A sub-section
+    `````````````