Add tips on porting LLMs from HuggingFace (#523)

* Add tips on porting LLMs from HuggingFace

* Add CONTRIBUTING.md  to mlx-examples-llms

* Refactor imports and update comment in starcoder2.py

* Update llms/mlx_lm/models/starcoder2.py

Co-authored-by: Awni Hannun <awni.hannun@gmail.com>

* nits

* nits

---------

Co-authored-by: Awni Hannun <awni.hannun@gmail.com>
Co-authored-by: Awni Hannun <awni@apple.com>

CONTRIBUTING.md

@@ -14,11 +14,11 @@ possible.
  
    You can also run the formatters manually as follows:
  
-     ```
+     ```bash
      clang-format -i file.cpp
      ```
  
-     ```
+     ```bash
      black file.py
      ```