updating README

README.rdoc

@@ -139,6 +139,22 @@ This package also includes the +shopify+ executable to make it easy to open up a
     shopify help
 
 
+== Threadsafety
+
+ActiveResource is inherently non-threadsafe, because class variables like `ActiveResource::Base.site` and
+`ActiveResource::Base.headers` are shared between threads. This can cause conflicts when using
+threaded libraries, like Sidekiq.
+
+We have a forked version of ActiveResource that stores these class variables in threadlocal
+variables. Using this forked version will allow ShopifyAPI to be used in a threaded environment.
+
+To enable threadsafety with ShopifyAPI, add the following to your Gemfile:
+
+```
+gem 'activeresource', :git => 'git://github.com/peterjm/activeresource', :ref => '47e7f181d9'
+gem 'shopify_api', '>= 3.3.0'
+```
+
 == Using Development Version
 
 Download the source code and run: