Updated README

README.md

+tf_mod_aws_iam_steamhaus_role
+==============
+A Terraform module for creating the Steamhaus role that can be assumed by users from the Steamhaus root account.
+The role assumes that the account that is being configured has an External ID associated with it and the user that will be assuming the role has MFA enabled. 
+
+Input Variables
+---------------
+
+- `source_account_id` - The ID of the Steamhaus root account, this defaults to 153730384924.
+- `external_id` - The External ID associated with the customer account.
+
+Outputs
+-------
+
+- `role_name` - The name of the role
+- `role_arn` - The ARN of the role
+
+Usage
+-----
+
+You can use these in your terraform template with the following steps.
+
+1.) Adding a module resource to your template, e.g. `main.tf`
+
+```
+#############################################################################################################
+# IAM
+#############################################################################################################
+module "steamhaus" {
+  source = "git::https://git.steamhaus.co.uk/steamhaus/tf_mod_aws_iam_steamhaus_role"
+  external_id = "${var.external_id}"
+}
+```
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