Add environment_vars documentation

Author: syndbg

README.md

@@ -71,6 +71,9 @@ provisioner         :
 
     ## When running on EC2 with Windows and using get-password pass the password as ansible_password variable
     pass_transport_password: false
+    ## (optional), if you want to set specific environment variables when running ansible
+    environment_vars:
+	PROXMOX_URL: https://example.com:8006
 ```
 ## Idempotency test
 If you want to check your code is idempotent you can use the idempotency_test. Essentially, this will run Ansible twice and check nothing changed in the second run. If something changed it will list the tasks. Note: If your using Ansible callback in your config this might conflict.