README: add OpenRouter instructions (#282)

README: Add instructions for using OpenRouter.

README.org

@@ -20,6 +20,7 @@ GPTel is a simple Large Language Model chat client for Emacs, with support for m
 | 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]]                          |
+| OpenRouter         | ✓      | [[https://openrouter.ai/keys][API key]]                          |
 
 *General usage*: ([[https://www.youtube.com/watch?v=bsRnh_brggM][YouTube Demo]])
 
@@ -61,6 +62,7 @@ GPTel uses Curl if available, but falls back to url-retrieve to work without ext
       - [[#perplexity][Perplexity]]
       - [[#anthropic-claude][Anthropic (Claude)]]
       - [[#groq][Groq]]
+      - [[#openrouter][OpenRouter]]
   - [[#usage][Usage]]
     - [[#in-any-buffer][In any buffer:]]
     - [[#in-a-dedicated-chat-buffer][In a dedicated chat buffer:]]
@@ -506,6 +508,54 @@ The above code makes the backend available to select.  If you want it to be the
 
 #+html: </details>
 
+#+html: <details><summary>
+**** OpenRouter
+#+html: </summary>
+
+Register a backend with
+#+begin_src emacs-lisp
+;; OpenRouter offers an OpenAI compatible API
+(gptel-make-openai "OpenRouter"               ;Any name you want
+  :host "openrouter.ai"
+  :endpoint "/api/v1/chat/completions"
+  :stream t
+  :key "your-api-key"                   ;can be a function that returns the key
+  :models '("openai/gpt-3.5-turbo"
+            "mistralai/mixtral-8x7b-instruct"
+            "meta-llama/codellama-34b-instruct"
+            "codellama/codellama-70b-instruct"
+            "google/palm-2-codechat-bison-32k"
+            "google/gemini-pro"))
+
+#+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   "mixtral-8x7b-32768"
+      gptel-backend
+      (gptel-make-openai "OpenRouter"               ;Any name you want
+        :host "openrouter.ai"
+        :endpoint "/api/v1/chat/completions"
+        :stream t
+        :key "your-api-key"                   ;can be a function that returns the key
+        :models '("openai/gpt-3.5-turbo"
+                  "mistralai/mixtral-8x7b-instruct"
+                  "meta-llama/codellama-34b-instruct"
+                  "codellama/codellama-70b-instruct"
+                  "google/palm-2-codechat-bison-32k"
+                  "google/gemini-pro")))
+
+#+end_src
+
+#+html: </details>
+
+
+
 ** Usage
 
 (This is also a [[https://www.youtube.com/watch?v=bsRnh_brggM][video demo]] showing various uses of gptel.)