add openai env example and update README (#764)

.env.example

@@ -6,6 +6,10 @@ ENV=local
 ENABLE_OPENAI=false
 # OPENAI_API_KEY: Your OpenAI API key for accessing models like GPT-4.
 OPENAI_API_KEY=""
+# OPENAI_API_BASE: Your OpenAI API Base url. Optional.
+# OPENAI_API_BASE=""
+# OPENAI_ORGANIZATION: Your OpenAI org-id. Optional.
+# OPENAI_ORGANIZATION=""
 
 # ENABLE_ANTHROPIC: Set to true to enable Anthropic as a language model provider.
 ENABLE_ANTHROPIC=false