Deprecate engine cases

[zcutlerf]

Made a few changes here. Please let me know if any of these could be reworked. -Deprecated the enum cases, which produces some warnings in the package. I haven't figured out how to "deprecate in the future." -After re-reading OpenAI's documentation, it seems like Completion should be used with the newer Instruct Series, and the other methods should be used with the older Base Series. I've streamlined the readme to reflect this. -I also moved the extension suggestion higher in the readme so it's more noticeable.