Related to #1615
Add documentation and function for exporting models from Colab to local machines.
* **README.md**: Add a new section titled "Exporting Models from Colab to Local Machine" under "✨ Finetune for Free" with detailed steps for exporting models from Colab to local machines.
* **CONTRIBUTING.md**: Add a note about the new documentation section for exporting models from Colab.
* **unsloth/save.py**: Add a new function `export_model_to_local` to handle exporting models from Colab to local machines.
(cherry picked from commit 0361bd658f)
