Add deprecation doc

Author: jseabold

doc/source/whatsnew/v1.2.0.rst

@@ -291,6 +291,7 @@ Deprecations
 - :meth:`Rolling.count` with ``min_periods=None`` will default to the size of the window in a future version (:issue:`31302`)
 - Deprecated slice-indexing on timezone-aware :class:`DatetimeIndex` with naive ``datetime`` objects, to match scalar indexing behavior (:issue:`36148`)
 - :meth:`Index.ravel` returning a ``np.ndarray`` is deprecated, in the future this will return a view on the same index (:issue:`19956`)
+- Deprecated default keyword argument of ``observed=False`` in :~meth:`DataFrame.groupby` and :~meth:`DataFrame.pivot_table` (:issue:`17594`)
 
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