example in README is outdated

[Thomas Friedel]

when trying to run the example from the README
the line "from promptimize.prompts import SimplePrompt" can not be executed because:
ModuleNotFoundError: No module named 'promptimize.prompts'

It seems like the README was written for an earlier version and meanwhile the code has been refactored.

[Edward Ho]

Tried to address in my PR. No response so far.