README: Instructions for Mistral Le Chat (#766)

README.org: Add instructions for Mistral Le Chat,
 addressing issue #703.

Author: ligon

README.org

@@ -21,6 +21,7 @@ gptel is a simple Large Language Model chat client for Emacs, with support for m
 | Kagi Summarizer    | ✓        | [[https://kagi.com/settings?p=api][API key]]                    |
 | Azure              | ✓        | Deployment and API key     |
 | Groq               | ✓        | [[https://console.groq.com/keys][API key]]                    |
+| Mistral Le Chat    | ✓        | [[https://console.mistral.ai/api-keys][API key]]                    |
 | Perplexity         | ✓        | [[https://docs.perplexity.ai/docs/getting-started][API key]]                    |
 | OpenRouter         | ✓        | [[https://openrouter.ai/keys][API key]]                    |
 | together.ai        | ✓        | [[https://api.together.xyz/settings/api-keys][API key]]                    |
@@ -104,6 +105,7 @@ gptel uses Curl if available, but falls back to the built-in url-retrieve to wor
       - [[#perplexity][Perplexity]]
       - [[#anthropic-claude][Anthropic (Claude)]]
       - [[#groq][Groq]]
+      - [[#mistral-le-chat][Mistral Le Chat]]
       - [[#openrouter][OpenRouter]]
       - [[#privategpt][PrivateGPT]]
       - [[#deepseek][DeepSeek]]
@@ -603,6 +605,41 @@ The above code makes the backend available to select.  If you want it to be the
 #+html: </details>
 
 #+html: <details><summary>
+**** Mistral Le Chat
+#+html: </summary>
+
+Register a backend with
+#+begin_src emacs-lisp
+;; Mistral offers an OpenAI compatible API
+(gptel-make-openai "MistralLeChat"  ;Any name you want
+  :host "api.mistral.ai"
+  :endpoint "/v1/chat/completions"
+  :protocol "https"
+  :key "your-api-key"               ;can be a function that returns the key
+  :models '("mistral-small"))
+#+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   'mistral-small
+      gptel-backend
+      (gptel-make-openai "MistralLeChat"  ;Any name you want
+        :host "api.mistral.ai"
+        :endpoint "/v1/chat/completions"
+        :protocol "https"
+        :key "your-api-key"               ;can be a function that returns the key
+        :models '("mistral-small")))
+#+end_src
+
+#+html: </details>
+
+#+html: <details><summary>
+
 **** OpenRouter
 #+html: </summary>