DOC: new mini-section on specifying formats on datetime parsing

[jreback]

We have a single note on how to use format= kw in timeseries.rst: https://github.com/pandas-dev/pandas/blob/master/doc/source/timeseries.rst#L178

I think let's make this a min-sub-section and provide links (or maybe just replicate) python strptime docs and show how to use the codings for specifying formats in some examples.

[manrajgrover]

@jreback I can take this one up as a start to contribute to pandas.