README: Add support for Groq (#257)

* README.org: Mention details for configuring Groq.

README.org

@@ -19,6 +19,7 @@ GPTel is a simple Large Language Model chat client for Emacs, with support for m
 | Anyscale           | ✓      | [[https://docs.endpoints.anyscale.com/][API key]]                   |
 | Perplexity         | ✓      | [[https://docs.perplexity.ai/docs/getting-started][API key]]                   |
 | Anthropic (Claude) | ✓      | [[https://www.anthropic.com/api][API key]]                   |
+| Groq               | ✓      | [[https://console.groq.com/keys][API key]]                          |
 
 *General usage*: ([[https://www.youtube.com/watch?v=bsRnh_brggM][YouTube Demo]])
 
@@ -59,6 +60,7 @@ GPTel uses Curl if available, but falls back to url-retrieve to work without ext
       - [[#anyscale][Anyscale]]
       - [[#perplexity][Perplexity]]
       - [[#anthropic-claude][Anthropic (Claude)]]
+      - [[#groq][Groq]]
   - [[#usage][Usage]]
     - [[#in-any-buffer][In any buffer:]]
     - [[#in-a-dedicated-chat-buffer][In a dedicated chat buffer:]]
@@ -465,6 +467,44 @@ The above code makes the backend available to select.  If you want it to be the
                  :stream t :key "your-api-key"))
 #+end_src
 
+#+html: </details>
+#+html: <details><summary>
+**** Groq
+#+html: </summary>
+
+Register a backend with
+#+begin_src emacs-lisp
+;; Groq offers an OpenAI compatible API
+(gptel-make-openai "Groq"               ;Any name you want
+  :host "api.groq.com"
+  :endpoint "/openai/v1/chat/completions"
+  :stream t
+  :key "your-api-key"                   ;can be a function that returns the key
+  :models '("mixtral-8x7b-32768"
+            "gemma-7b-it"
+            "llama2-70b-4096"))
+#+end_src
+
+You can pick this backend from the menu when using gptel (see [[#usage][Usage]]).  Note that Groq is fast enough that you could easily set =:stream nil= and still get near-instant responses.
+
+***** (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 default value of =gptel-backend=.  Use this instead of the above.
+#+begin_src emacs-lisp
+;; OPTIONAL configuration
+(setq-default
+ gptel-model   "mixtral-8x7b-32768"
+ gptel-backend
+ (gptel-make-openai "Groq"
+  :host "api.groq.com"
+  :endpoint "/openai/v1/chat/completions"
+  :stream t
+  :key "your-api-key"
+  :models '("mixtral-8x7b-32768"
+            "gemma-7b-it"
+            "llama2-70b-4096")))
+#+end_src
+
 #+html: </details>
 
 ** Usage