README: Add support for llama.cpp

* README.org: The llama.cpp server supports OpenAI's API, so we
can reuse it.  Closes #121.

README.org

@@ -5,14 +5,14 @@
 GPTel is a simple Large Language Model chat client for Emacs, with support for multiple models/backends.
 
 | LLM Backend | Supports | Requires                  |
-|-------------+----------+-------------------------|
+|-------------+----------+---------------------------|
 | ChatGPT     | ✓       | [[https://platform.openai.com/account/api-keys][API key]]                   |
 | Azure       | ✓       | Deployment and API key    |
 | Ollama      | ✓       | [[https://ollama.ai/][Ollama running locally]]    |
 | GPT4All     | ✓       | [[https://gpt4all.io/index.html][GPT4All running locally]]   |
 | Gemini      | ✓       | [[https://makersuite.google.com/app/apikey][API key]]                   |
+| Llama.cpp   | ✓       | [[https://github.com/ggerganov/llama.cpp/tree/master/examples/server#quick-start][Llama.cpp running locally]] |
 | PrivateGPT  | Planned  | -                         |
-| Llama.cpp   | Planned  | -                       |
 
 *General usage*: ([[https://www.youtube.com/watch?v=bsRnh_brggM][YouTube Demo]])
 
@@ -46,6 +46,7 @@ GPTel uses Curl if available, but falls back to url-retrieve to work without ext
       - [[#gpt4all][GPT4All]]
       - [[#ollama][Ollama]]
       - [[#gemini][Gemini]]
+      - [[#llamacpp][Llama.cpp]]
   - [[#usage][Usage]]
     - [[#in-any-buffer][In any buffer:]]
     - [[#in-a-dedicated-chat-buffer][In a dedicated chat buffer:]]
@@ -221,6 +222,30 @@ You can pick this backend from the transient menu when using gptel (see Usage),
 
 #+html: </details>
 
+#+html: <details>
+#+html: <summary>
+**** Llama.cpp
+#+html: </summary>
+
+Register a backend with
+#+begin_src emacs-lisp
+(gptel-make-openai                    ;Not a typo, same API as OpenAI
+ "llama-cpp"                          ;Any name
+ :stream t                            ;Stream responses
+ :protocol "http"
+ :host "localhost:8000"               ;Llama.cpp server location
+ :models '("test"))                   ;List of available models
+#+end_src
+These are the required parameters, refer to the documentation of =gptel-make-openai= for more.
+
+You can pick this backend from the transient menu when using gptel (see Usage), or set this as the default value of =gptel-backend=:
+#+begin_src emacs-lisp
+(setq-default gptel-backend (gptel-make-openai "llama-cpp" ...)
+              gptel-model   "test")
+#+end_src
+
+#+html: </details>
+
 ** Usage
 
 (This is also a [[https://www.youtube.com/watch?v=bsRnh_brggM][video demo]] showing various uses of gptel.)