Add simple classification tutorial to readme
* [Classification Tutorial](classification_tutorial.ipynb)
* Here is an example for CatBoost to solve binary classification and multi-classification problems.
+* [Simple classification example with missing feature handling and parameter tuning](classification_with_parameter_tuning_tutorial.ipynb)
+ * Tutorial shows you how to use CatBoost to train binary classifier for data with missing features and how to do hyper-parameter tuning using [Hyperopt](https://github.com/hyperopt/hyperopt) framework.
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