added set_winrm_passwd usage instructions

website/source/docs/provisioners/ansible.html.md

@@ -128,6 +128,11 @@ Optional Parameters:
 -   `user` (string) - The `ansible_user` to use. Defaults to the user running
     packer.
 
+-   `set_winrm_passwd` (boolean) - Set to `true` to access the randomly-generated 
+    EC2 Windows password through the environment variable `GENERATED_WINRM_PASSWORD`.
+    You will also need to set `ansible_password` in your ansible playbook, for example,
+    `ansible_password: "{{ lookup('env','GENERATED_WINRM_PASSWORD') }}"`
+
 ## Default Extra Variables
 
 In addition to being able to specify extra arguments using the