Add explanatory notes to the CONTRIBUTING.md file

[TheNourhan]

Description

I added explanatory notes to the contribution file, related to .env file and emulator.

Motivation and Context

Adding these lines will help the contributors and save their time. I took a long time to install the project because these notes were not explicitly explained.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.