Added README.md files to describe about contents of the directory.

PiperOrigin-RevId: 438078675

Author: tensorflower-gardener

official/legacy/README.md

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+Models in this `legacy` directory are mainly are used for benchmarking the
+models.
+
+Please note that the models in this `legacy` directory are not supported like
+the models in official/nlp and official/vision.

official/nlp/data/README.md

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+This directory contains binaries and utils required for input preprocessing,
+tokenization, etc that can be used with model building blocks available in
+NLP modeling library [nlp/modelling]
+(https://github.com/tensorflow/models/tree/master/official/nlp/modeling) to
+train custom models and validate new research ideas.