Merge branch '4.2'

* 4.2:
  Add CORS_ALLOWED_ORIGINS to running mercure

Author: javiereguiluz

mercure.rst

@@ -71,7 +71,7 @@ Run the following command to start it:
 
 .. code-block:: terminal
 
-    $ JWT_KEY='aVerySecretKey' ADDR='localhost:3000' ALLOW_ANONYMOUS=1 ./mercure
+    $ JWT_KEY='aVerySecretKey' ADDR='localhost:3000' ALLOW_ANONYMOUS=1 CORS_ALLOWED_ORIGINS=* ./mercure
 
 .. note::