DOC: added a reference to DataFrame assign in concatenate section of merging

Because of various stack overflow answers we are directed to the merging
page where a link to DataFrame.assign would be better. Adding the
reference here will make people's code better. At least it would have
for me.

Author: obilodeau

doc/source/merging.rst

@@ -303,6 +303,28 @@ This is also a valid argument to ``DataFrame.append``:
           labels=['df1', 'df4'], vertical=True);
    plt.close('all');
 
+.. _merging.assign:
+
+Concatenating using ``assign``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can concatenate one Series to a DataFrame using the DataFrame method
+:meth:`~pandas.DataFrame.assign`. This will add a column to the DataFrame using
+the Series' data. New columns can be potentially derived from existing columns.
+
+.. ipython:: python
+
+   s1 = pd.Series(['X0', 'X1', 'X2', 'X3'])
+   result = df1.assign(X = s1)
+
+.. ipython:: python
+   :suppress:
+
+   @savefig merging_assign.png
+   p.plot([df1, s1], result,
+          labels=['df1', 's1'], vertical=False);
+   plt.close('all');
+
 .. _merging.mixed_ndims:
 
 Concatenating with mixed ndims

doc/source/whatsnew/v0.21.1.txt

@@ -48,7 +48,7 @@ Performance Improvements
 Documentation Changes
 ~~~~~~~~~~~~~~~~~~~~~
 
--
+- Added a reference to :func:`DataFrame.assign` in the concatenate section of the merging documentation (:issue:`18665`)
 -
 -