Warn in release notes that internal groups can't contain external groups

Since the abstraction of the group system into the GroupBackend
interface, it is not possible anymore to have external groups as
members of internal groups. Add a warning about this to the Gerrit 2.5
release notes.

Change-Id: I076201d07b67bb63893f553169268d543ebf0a1d
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/ReleaseNotes/ReleaseNotes-2.5.txt b/ReleaseNotes/ReleaseNotes-2.5.txt
index 3bdf986..124fe25 100644
--- a/ReleaseNotes/ReleaseNotes-2.5.txt
+++ b/ReleaseNotes/ReleaseNotes-2.5.txt
@@ -1071,6 +1071,9 @@
 needs to be prefixed with the `ldap/` string. E.g. if there is a group
 in LDAP which is called "Developers", Gerrit will suggest this group
 when the user types `ldap/De`.
++
+WARNING: External groups are not anymore allowed to be members of
+internal groups.
 
 [[migrate-ldap-groups]]
 * Migrate existing internal LDAP groups