Add note about reading OPENAI_KEY and OPENAI_API_KEY variables

[WiegerWolf]

Today I've spent good 30 min trying to figure out why my configured OPENAI_KEY from ~/.aicommits was not being used. It turned out env vars OPENAI_KEY and OPENAI_API_KEY take priority over config file. This is second time this happened to me, and each time I had to go and read the code.

While this behaviour is more or less fine, I'd like to add this note to README explaining this behaviour to users.

[WiegerWolf]

tests fail with:

process.env.OPENAI_KEY is necessary to run these tests. Skipping...

and even tough it says "Skipping...", in the end the run fails with:

944ms
14 passed

 ELIFECYCLE  Test failed. See above for more details.
Error: Process completed with exit code 1.

I'm not sure how to proceed, but the pipeline failing has nothing to do with the changes in this PR.