Add description about environment to Contributing Guidelines

**Link to the document**
https://github.com/aws/studio-lab-examples/blob/main/CONTRIBUTING.md

**What aspects of the notebook can be improved?**
For now, Contributing Guidelines does not mention about how to create the environment. For that reason, the way to prepare for the environment is different one by one.

* [Use pip install in notebook](https://github.com/aws/studio-lab-examples/blob/main/computer-vision/weather-data/weather-image-classification-pytorch.ipynb)
* [Use environment.yml](https://github.com/aws/studio-lab-examples/blob/main/computer-vision/kmnist/cv-kminst.ipynb)

**What are your suggestions?**
We should mention about how to prepare the environment in the Contributing Guidelines.
I think conda and pip have both Pros and Cons. For that reason, it will be good that contributor can choose which to use depend on the material.

For example,

* Level1 (required): prepare the set up instruction by conda or pip.
* option1: Prepare the `environment.yml` and describe how to use it in the notebook
* option2: Execute `pip install` inside the notebook and mention about how to reset (recover) the environment.
* Lavel2 (optional): mention about both way (option1 & option2)

If you have some opinions about it, please comment on this issue!

Start with CONTRIBUTING.md, then compare the linked weather-image-classification-pytorch.ipynb and cv-kminst.ipynb examples to understand their environment setup. Done means the Contributing Guidelines explain how contributors should prepare an environment, including the relevant setup approaches and recovery guidance.

Written by the indexing model from the issue text.