Add configuration for OpenRouter to README

Author: khasinski

README.md

@@ -103,6 +103,7 @@ RubyLLM.configure do |config|
   config.anthropic_api_key = ENV['ANTHROPIC_API_KEY']
   config.gemini_api_key = ENV['GEMINI_API_KEY']
   config.deepseek_api_key = ENV['DEEPSEEK_API_KEY']
+  config.openrouter_api_key = ENV['OPENROUTER_API_KEY']
 end
 ```