DOC: Add append example in to_excel documentation (pandas-dev#27852)

Author: sameshl

pandas/core/generic.py

@@ -2179,6 +2179,12 @@ def _repr_data_resource_(self):
     ...     df1.to_excel(writer, sheet_name='Sheet_name_1')
     ...     df2.to_excel(writer, sheet_name='Sheet_name_2')
 
+    ExcelWriter can also be used to append to an existing Excel file:
+
+    >>> with pd.ExcelWriter('output.xlsx',
+    ...                     mode='a') as writer:  # doctest: +SKIP
+    ...     df.to_excel(writer, sheet_name='Sheet_name_3')
+
     To set the library that is used to write the Excel file,
     you can pass the `engine` keyword (the default engine is
     automatically chosen depending on the file extension):