Explain warning for schema version 68 in Gerrit 2.5 release notes

The migration for schema version 68 may result in a warning. Explain
in the Gerrit 2.5 release notes why this warning may happen and in
which case it can be safely ignored.

Change-Id: I5cdb392e5e3b7389818ad0d57bc1390d3ab408f6
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
1 file changed