DOC: add cookbook example
BUG: (GH4532) Fix bug in having a rhs of np.timedelta64 or np.offsets.DateOffset when operating
     with datetimes
BUG: (GH4134) Fix arithmetic with series/datetimeindex and np.timedelta64 not working the same

TST: add in @cpcloud tests related to GH4135
ENH: GH4521 A Series of dtype timedelta64[ns] can now be divided/multipled by an integer series
DOC/ENH: timedelta conversions & docs