adding firebase_options instructions to contributing.md

[patched-codes[bot]]

## File Changed: `CONTRIBUTING.md` Details: Potential security vulnerability introduced by including a link to Firebase configuration instructions without emphasizing the importance of keeping configuration files secure. Affected Code Snippet: ```bash #### Add firebase configurations Follow [this link](https://firebase.google.com/docs/flutter/setup) for creating firebase configurations file in flutter ``` Start Line: 53 End Line: 54