diff --git a/security/guardduty/README.md b/security/guardduty/README.md
index addbb1d..a968e1b 100644
--- a/security/guardduty/README.md
+++ b/security/guardduty/README.md
@@ -1,9 +1,11 @@
 # Guard Duty
 
-This modules sets up Guard Duty in the given regions.
+This modules sets up Guard Duty in specific AWS regions (all, by default).
 
 https://aws.amazon.com/guardduty/
 
+Guard Duty is setup per-region which means for a Terraform project you need to specify providers for all regions and source a given module for each region. This module uses Cloudformation Stack Sets under the hood to simplify the deployment so that one module sourcing can provision Guard Duty in all regions.
+
 ## Usage
 
 ```
@@ -12,6 +14,11 @@ module "guardduty" {
 }
 ```
 
+## Limitations
+
+Due to the way Terraform interfaces with Cloudformation Stack Set instances the module cannot apply resources in parallel - the stack set is busy. So the only limitation with this module is that you must run Terraform with the `-parellism=1` flag set.
+
+
 ## Inputs
 
 You don't need to provide any inputs, but you can override all of the defaults.