Added ConfBridge AMI event note to UPGRADE.txt.
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Merged revisions 422255 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@422256 f38db490-d61c-443f-a65b-d21fe96a405b