README.md in active_zuora-2.1.0 vs README.md in active_zuora-2.1.1

@@ -7,9 +7,12 @@
 ## Active Zuora Version 1
 This repostiory contains >= Version 2 of Active Zuora
 
 Version 1 can be found at https://github.com/sportngin/active_zuora_v1
 
+## Thread Safety
+2.0.X versions of Active Zuora are not thread safe. They depend on a version of [Savon](https://github.com/savonrb/savon) that is [does not work with threads](https://github.com/savonrb/savon/issues/259).
+
 ## Configuration
 
     ActiveZuora.configure(
       :username => '[e-mail]',
       :password => 'password'