BUG: Allow duplicate indices when performing operations that align (GH5185)
ENH: Add date_format keyword to to_csv()

DOC: add date_format to release notes
DOC: minor whatsnew changes
Merge pull request #5187 from jreback/dup_ops

BUG: Allow duplicate indices when performing operations that align (GH5185)
DOC: whatsnew changes
Merge branch 'to_csv_date_format' of https://github.com/qwhelan/pandas into qwhelan-to_csv_date_format

Conflicts:
	doc/source/v0.13.0.txt