add notes to docs about how to use --extra-vars in ansible for transient cloud spin up

README

@@ -47,6 +47,21 @@ volumes, etc) and it's ip address.
 
 You will then be able to login, as root.
 
+You can add various extra vars to the above commands to change the instance
+you've just spun up.
+
+variables to define:
+instance_type=c1.medium
+security_group=default
+root_auth_users='username1 username2 @groupname'
+hostbase=basename for hostname - will have instance id appended to it
+
+
+define these with:
+
+--extra-vars="varname=value varname1=value varname2=value"
+
+
 
 
 Instance types: right now the euca cloudlet has these instance types defined