Update README.md on excluding subscriptions

Author: Rado Marina

README.md

@@ -56,6 +56,10 @@ Choose one of the following installation options based on your needs:
 
 Use this option if you want Firefly to automatically discover and monitor all accessible Azure subscriptions.
 
+You can exclude subscriptions by adding a tag on them.
+
+```disable_firefly_discovery = "Enabled"```
+
 ```hcl
 provider "azuread" {
   client_id     = var.client_id
@@ -185,4 +189,4 @@ When contributing, please:
 
 If you encounter any issues or have questions about using this repository, please open an issue on GitHub or contact Firefly support.
 
-For more information about Firefly and our services, please visit [our website](https://www.gofirefly.io/).
+For more information about Firefly and our services, please visit [our website](https://www.gofirefly.io/).