chore(README.md): update information about default model and cost

The default model used by OpenCommit is ChatGPT (3.5-turbo-16k) official model. It is mentioned that the cost should not exceed $0.10 per casual working day. Additionally, it is now mentioned that users have the option to switch to gpt-4, which is better but more expensive. This update provides users with more information about the available models and their associated costs.

Author: di-sukharev

README.md

@@ -332,4 +332,6 @@ You pay for your requests to OpenAI API on your own.
 
 OpenCommit stores your key locally.
 
-OpenCommit by default uses ChatGPT (3.5-turbo-16k) official model, which is a lot cheaper than gpt-4.
+OpenCommit by default uses 3.5-turbo-16k model, it should not exceed $0.10 per casual working day.
+
+You may switch to gpt-4, it's better, but more expensive.