docs: document BINANCE_BASE_URL env var

Author: monicatu333

.env.example

@@ -15,6 +15,11 @@ MISTRAL_API_KEY=
 MOONSHOT_API_KEY=
 GROQ_API_KEY=
 NVIDIA_API_KEY=
+# Optional: override Binance API base URL. Needed if you're in a region
+# where api.binance.com is restricted (e.g. the US) — use
+# https://api.us.binance.com/api/v3/klines instead. Leave unset to use
+# the default global endpoint.
+#BINANCE_BASE_URL=
 
 # FRED (Federal Reserve macro data: rates, inflation, labor, growth). Free key: https://fred.stlouisfed.org/docs/api/api_key.html
 #FRED_API_KEY=