README.md: add warning about reverting changes

README.md

@@ -7,6 +7,11 @@ language (or before improving/updating the translation files).
 In short, it will update most CiviCRM option group and option value titles,
 as well as profiles, by attempting to translate the strings.
 
+IMPORTANT: You cannot revert the process. If you want to switch back to English
+afterwards, you will have to translate your configurations manually (or you could
+enabled [detailed logging](https://docs.civicrm.org/sysadmin/en/latest/setup/logging/),
+but that still requires a fair amount of fiddling in SQL.
+
 Currently the extension does not handle multi-lingual sites. It might work if
 you enable the extension from the CiviCRM Extension Manager (web interface),
 while your interface is in the target language. It has not been tested yet.