Add note to README about katello systems

README.md

@@ -21,6 +21,16 @@ Running this script will convert an existing CentOS 8 system to Rocky Linux 8.
 
 ### Known Issues
 
+#### Katello clashes
+
+Katello installs it's own repositories via subscription-manager that are meant
+to replace the ones from CentOS, but it doe snot remove, or even disable, the
+corresponding CentOS repositories.  This would cause issues to a normal running
+system, but it is especially problematic for migrate2rocky as it means that
+migrate2rocky cannot properly determine which repositories to remove and
+attempting to run migrate2rocky on a katello will likely result in a corrupted
+system.
+
 #### RHEL migrations show error messages during conversion
 
 ```