Update readme

[rickbeeloo]

I was a bit confused by:

https://github.com/ShelbyJenkins/llm_client/blob/ed1c68a501a561548530dabd6019c2c561619328/src/llm_backends/llama_cpp/mod.rs#L27

As that path does not exist in the repo, so maybe adding git clone https://github.com/ggerganov/llama.cpp in the README clarifies that.

(Unrelated, cool package!❤️)