diff --git a/README b/README
index 77b37f8..bfbc760 100644
--- a/README
+++ b/README
@@ -134,6 +134,11 @@ A source definition is a dictionnary containing the following keys:
       retrieved for the current user,
     - `signature_key`, when using the HMAC authentication mechanism it
       holds the secret key used to sign the exchanges,
+    - `async`, if True make refreshing the cache asynchronous (using a thread),
+      beware that if the cache is currently empty a synchronous update will be
+      done, lock are used to limit update thread to one by URL, but it you use
+      a worker engine their could be multiple thread trying to update the same
+      cache in different workers, value is optional and its default is False,
     - `timeout`, a timeout for making the HTTP request, it is optional
       and it default to 10 seconds,
     - `refresh`, how long to cache the parsed value of the source, it