Update README.md

[kevinkhill]

Adding -0613 to the model per OpenAI stating that those models have the function calling capabilities.

[kevinkhill]

I don't know if you want to provide a constant for end-users, but this is how I keep it straight in my projects:

export const GPT_35_TURBO = "gpt-3.5-turbo";
export const GPT_35_TURBO_WITH_FNS = `${GPT_35_TURBO}-0613`;