Document the expected API key Environment Variable names for .env #86

Description of feature

I just wanted to try out the package, and it took me a while to find the expected API key Environment Variable names for the .env file. I finally found them running cell_annotator.APIKeyManager().print_status(), but it would be great if they are also documented, e.g. in the README. Or at least maybe pointing to cell_annotator.APIKeyManager().print_status() in the README?