Add contributing.md to keras_nlp.

PiperOrigin-RevId: 331676499

official/nlp/keras_nlp/contributing.md

+## Contributing to KerasNLP
+
+Patches to KerasNLP are welcome!
+
+The source-of-truth repository lives under
+[TF Model Garden NLP](https://github.com/tensorflow/models/official/nlp/keras_nlp),
+and is mirrored as a read-only repository under
+[keras-team/keras-nlp](https://github.com/keras-team/keras-nlp).
+Contributions should be made as PRs to the TF Model Garden repository.
+This is to ensure the codebase is rigorously tested with state-of-art models
+on different accelerators.
+In the long run, we will move development to the current repository `keras-team/keras-nlp`.
+
+## :heavy_check_mark: Contributor checklist
+
+1. Ensure you have signed the [Contributor License Agreement](https://cla.developers.google.com/about/google-individual?csw=1).
+    * All code contributors are required to sign a Contributor License Agreement.
+    * Please read this [troubleshooting guide](Contributor-License-Agreements#troubleshooting-clas)
+    if you encounter an issue.
+2. Please review the [contribution guidelines](https://github.com/tensorflow/models/wiki/How-to-contribute).
+3. Check if your changes are consistent with the [TensorFlow coding style](https://www.tensorflow.org/community/contribute/code_style).