[doc] Remove upgrade warning since it seems to be solved

README.md

@@ -28,17 +28,10 @@ or from the Web administration:
 
 ## Upgrade
 
-The upgrade from the current official package seems works, BUT:
-
- * ownCloud 9.0 comes with a rewrite **Calendar** application, which doesn't seem to import your
-   calendar(s). **Take care of exporting them first before doing the upgrade to not lose them!**
-
-You can upgrade to this package from the command line, using:
+You can upgrade to this package from the command line for testing purpose with:
 
     $ sudo yunohost app upgrade -u https://github.com/jeromelebleu/owncloud_ynh/tree/testing owncloud
 
-Again, upgrading from the current official package should be used for testing only - or with care!
-
 ## TODO
 
  * Add a *clean* argument to the remove script to delete data folder?