docs: add environment variable example

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,8 @@
+# Telegram Bot Token from @BotFather
+FLYNN_TELEGRAM_TOKEN=your-bot-token-here
+
+# Anthropic API Key
+ANTHROPIC_API_KEY=your-anthropic-api-key-here
+
+# Optional: Custom config path
+# FLYNN_CONFIG=/path/to/config.yaml