Added time-series section and Zumbach&Muller's paper on inhomogeneous operators

time_series/README.md

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+Important papers relating to time-series data
+
+Time-series data presents specific but very common problems for efficient 
+analysis, resulting in the need for columnar data stores and iterative 
+one-pass processing.
+
+The included documents are:
+
+* [Operators on Inhomogeneous Time Series] (http://papers.ssrn.com/sol3/papers.cfm?abstract_id=208278) - Gilles O. Zumbach and Ulrich A. Müller
+
+  We present a toolbox to compute and extract information from
+  inhomogeneous (i.e. unequally spaced) time series. The toolbox
+  contains a large set of operators, mapping from the space of
+  inhomogeneous time series to itself.
+
+  These operators are computationally efficient (time and memory-wise)
+  and suitable for stochastic processes. This makes them attractive for
+  processing high-frequency data in finance and other fields. Using a
+  basic set of operators, we easily construct more powerful combined
+  operators which cover a wide set of typical applications. 
+
+
+