README: Add support for xAI (#466)

README.org: Add instructions for xAI.

Author: WuuBoLin

README.org

@@ -27,6 +27,7 @@ gptel is a simple Large Language Model chat client for Emacs, with support for m
 | Cerebras           | ✓        | [[https://cloud.cerebras.ai/][API key]]                    |
 | Github Models      | ✓        | [[https://github.com/settings/tokens][Token]]                      |
 | Novita AI          | ✓        | [[https://novita.ai/model-api/product/llm-api?utm_source=github_gptel&utm_medium=github_readme&utm_campaign=link][Token]]                      |
+| xAI                | ✓        | [[https://console.x.ai?utm_source=github_gptel&utm_medium=github_readme&utm_campaign=link][API key]]                    |
 #+html: </div>
 
 *General usage*: ([[https://www.youtube.com/watch?v=bsRnh_brggM][YouTube Demo]])
@@ -81,6 +82,7 @@ gptel uses Curl if available, but falls back to url-retrieve to work without ext
       - [[#cerebras][Cerebras]]
       - [[#github-models][Github Models]]
       - [[#novita-ai][Novita AI]]
+      - [[#xAI][xAI]]
   - [[#usage][Usage]]
     - [[#in-any-buffer][In any buffer:]]
     - [[#in-a-dedicated-chat-buffer][In a dedicated chat buffer:]]
@@ -770,6 +772,43 @@ The above code makes the backend available to select.  If you want it to be the
 
 #+html: </details>
 
+#+html: <details><summary>
+**** xAI
+#+html: </summary>
+
+Register a backend with
+#+begin_src emacs-lisp
+;; xAI offers an OpenAI compatible API
+(gptel-make-openai "xAI"           ;Any name you want
+  :host "api.x.ai"
+  :key "your-api-key"              ;can be a function that returns the key
+  :endpoint "/v1/chat/completions"
+  :stream t
+  :models '(;; xAI now only offers `grok-beta` as of the time of this writing
+            grok-beta))
+#+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   'grok-beta
+ gptel-backend
+ (gptel-make-openai "xAI"           ;Any name you want
+   :host "api.x.ai"
+   :key "your-api-key"              ;can be a function that returns the key
+   :endpoint "/v1/chat/completions"
+   :stream t
+   :models '(;; xAI now only offers `grok-beta` as of the time of this writing
+             grok-beta)))
+#+end_src
+
+#+html: </details>
+
 ** Usage
 
 (There is also a [[https://www.youtube.com/watch?v=bsRnh_brggM][video demo]] showing various uses of gptel.)