Add a colab to introduce nlp/modeling library with examples showing how to...

Add a colab to introduce nlp/modeling library with examples showing how to build pretrain/span labeling/classification models.

PiperOrigin-RevId: 320147054

official/nlp/modeling/README.md

 # NLP Modeling Library
 
-This libary provides a set of Keras primitives (Layers, Networks, and Models)
+This library provides a set of Keras primitives (Layers, Networks, and Models)
 that can be assembled into transformer-based models. They are
 flexible, validated, interoperable, and both TF1 and TF2 compatible.
 
@@ -16,6 +16,10 @@ standardized configuration.
 
 * [`losses`](losses) contains common loss computation used in NLP tasks.
 
+Please see the colab
+[Introduction_to_NLP_Modeling_Library.ipynb](https://colab.sandbox.google.com/github/tensorflow/models/blob/master/official/colab/nlp/nlp_modeling_library_intro.ipynb)
+for how to build transformer-based NLP models using above primitives.
+
 Besides the pre-defined primitives, it also provides scaffold classes to allow
 easy experimentation with noval achitectures, e.g., you don’t need to fork a whole Transformer object to try a different kind of attention primitive, for instance.