docs: add Premium license warning for externally-managed-workspace template (#397)

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
Co-authored-by: M Atif Ali <[email protected]>
Co-authored-by: Atif Ali <[email protected]>

Author: 3 people

registry/coder-labs/templates/externally-managed-workspace/README.md

@@ -8,6 +8,9 @@ tags: [external]
 
 # Externally Managed Workspace Template
 
+> [!IMPORTANT]
+> External agents require a [Premium](https://coder.com/pricing) Coder license.
+
 This template provides a minimal scaffolding for creating Coder workspaces that connect to externally provisioned compute resources.
 
 Use this template as a starting point to build your own custom templates for scenarios where you need to connect to existing infrastructure.