Add another case to the documentation of the missing Change-Id error

Gerrit rejects commits with 'missing Change-Id in commit message' if
the Change-Id is the only line in the commit message. Explain this case
in the Gerrit documentation.

Change-Id: I5700f72353456d080b8e9652cd7e7d00fd1d324e
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
1 file changed