Add note about .env files from parent folders

[endolith]

It was extremely confusing to me why an API key was being used within VSCode terminal that wasn't my OpenInterpreter key. This note might save other people time.

Describe the changes you have made:

Reference any relevant issues (e.g. "Fixes #000"):

Pre-Submission Checklist (optional but appreciated):

• [ ] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [ ] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [ ] Tested on Windows
• [ ] Tested on MacOS
• [ ] Tested on Linux