gptel: Add support for the DeepSeek API

* gptel.el: Mention instructions for DeepSeek and PrivateGPT in
the package description.

* README.org: Add instructions for the DeepSeek API.

Author: karthink

README.org

@@ -23,6 +23,7 @@ gptel is a simple Large Language Model chat client for Emacs, with support for m
 | Groq               | ✓      | [[https://console.groq.com/keys][API key]]                    |
 | OpenRouter         | ✓      | [[https://openrouter.ai/keys][API key]]                    |
 | PrivateGPT         | ✓      | [[https://github.com/zylon-ai/private-gpt#-documentation][PrivateGPT running locally]] |
+| DeepSeek           | ✓      | [[https://platform.deepseek.com/api_keys][API key]]                    |
 #+html: </div>
 
 *General usage*: ([[https://www.youtube.com/watch?v=bsRnh_brggM][YouTube Demo]])
@@ -67,6 +68,7 @@ gptel uses Curl if available, but falls back to url-retrieve to work without ext
       - [[#groq][Groq]]
       - [[#openrouter][OpenRouter]]
       - [[#privategpt][PrivateGPT]]
+      - [[#deepseek][DeepSeek]]
   - [[#usage][Usage]]
     - [[#in-any-buffer][In any buffer:]]
     - [[#in-a-dedicated-chat-buffer][In a dedicated chat buffer:]]
@@ -595,6 +597,41 @@ The above code makes the backend available to select.  If you want it to be the
 
 #+end_src
 
+#+html: </details>
+#+html: <details><summary>
+**** DeepSeek
+#+html: </summary>
+
+Register a backend with
+#+begin_src emacs-lisp
+;; DeepSeek offers an OpenAI compatible API
+(gptel-make-openai "DeepSeek"       ;Any name you want
+  :host "api.deepseek.com"
+  :endpoint "/chat/completions"
+  :stream t
+  :key "your-api-key"               ;can be a function that returns the key
+  :models '("deepseek-chat" "deepseek-coder"))
+
+#+end_src
+
+You can pick this backend from the menu when using gptel (see [[#usage][Usage]]).
+
+***** (Optional) Set as the default gptel backend
+
+The above code makes the backend available to select.  If you want it to be the default backend for gptel, you can set this as the value of =gptel-backend=.  Use this instead of the above.
+#+begin_src emacs-lisp
+;; OPTIONAL configuration
+(setq gptel-model   "deepseek-chat"
+      gptel-backend
+      (gptel-make-openai "DeepSeek"     ;Any name you want
+        :host "api.deepseek.com"
+        :endpoint "/chat/completions"
+        :stream t
+        :key "your-api-key"             ;can be a function that returns the key
+        :models '("deepseek-chat" "deepseek-coder")))
+
+#+end_src
+
 #+html: </details>
 
 ** Usage

gptel.el

@@ -32,7 +32,8 @@
 ;; gptel supports
 ;;
 ;; - The services ChatGPT, Azure, Gemini, Anthropic AI, Anyscale, Together.ai,
-;;   Perplexity, Anyscale, OpenRouter, Groq and Kagi (FastGPT & Summarizer)
+;;   Perplexity, Anyscale, OpenRouter, Groq, PrivateGPT, DeepSeek and Kagi
+;;   (FastGPT & Summarizer)
 ;; - Local models via Ollama, Llama.cpp, Llamafiles or GPT4All
 ;;
 ;;  Additionally, any LLM service (local or remote) that provides an
@@ -60,8 +61,9 @@
 ;; - For Gemini: define a gptel-backend with `gptel-make-gemini', which see.
 ;; - For Anthropic (Claude): define a gptel-backend with `gptel-make-anthropic',
 ;;   which see
-;; - For Together.ai, Anyscale, Perplexity, Groq and OpenRouter: define a
-;;   gptel-backend with `gptel-make-openai', which see.
+;; - For Together.ai, Anyscale, Perplexity, Groq, OpenRouter or DeepSeek: define
+;;   a gptel-backend with `gptel-make-openai', which see.
+;; - For PrivateGPT: define a backend with `gptel-make-privategpt', which see.
 ;; - For Kagi: define a gptel-backend with `gptel-make-kagi', which see.
 ;;
 ;; For local models using Ollama, Llama.cpp or GPT4All: