Add initial doc for text_normalization

docs/source/nlp/text_normalization.rst

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+.. _text_normalization:
+
+Text Normalization Models
+==========================
+Text normalization is the task of converting a written text into its spoken form. For example,
+``$123`` should be verbalized as ``one hundred twenty three dollars``, while ``123 King Ave``
+should be verbalized as ``one twenty three King Avenue``. Text normalization is typically used as
+a pre-processing step for a range of speech application such as text-to-speech synthesis (TTS).
+
+Data format
+------------------
+
+The data needs to be stored in TAB separated files (.tsv) with three columns, the first of which
+is the "semiotic class", the second is the input token and the third is the output. An example can
+be the dataset used in the `Google Text Normalization Challenge <https://www.kaggle.com/google-nlu/text-normalization>`_.