add NUT-19 example (#686)

.env.example

@@ -71,6 +71,12 @@ MINT_BACKEND_BOLT11_SAT=FakeWallet
 # MINT_BACKEND_BOLT11_USD=FakeWallet
 # MINT_BACKEND_BOLT11_EUR=FakeWallet
 
+# NUT-19 Cached responses
+# Enable these settings to cache responses in Redis
+#
+# MINT_REDIS_CACHE_ENABLED=TRUE
+# MINT_REDIS_CACHE_URL="redis://localhost:6379"
+
 # Funding source settings
 
 # Use with LndRPCWallet