allow dotenv in ai, add ai unit tests, add CONTRIBUTING.md

[rwmartinez]

I got tired of entering my API key every time I opened a new terminal and wrote some tests

[rwmartinez]

@patillacode @AntonOsika This is a great repo, thanks for the hard work maintaining it.

This MR adds the ability to load the OPENAI_API_KEY from .env instead of only via the CLI. It also separates app vs. dev dependencies and added contributing.md details. I saw you had .github/contributing.md in another PR so this should beef it up a bit.

[patillacode]

Open discussion related to this in #135

[AntonOsika]

Thanks @rwmartinez

For now, I prefer not using monkeypatch.

Also I think we are fine to just use env variables to keep it as simple as possible for now