json logging options

[Alex]

Hi,
I'm using amavisd-new-2.10.1 on fedora22 and would like to use the
json logging option:

$logline_maxlen = ( 32*1024 ) - 50; # 32k max message size, keep 50
bytes for syslog
$log_templ = <<'EOD';
[:report_json]
EOD

Is there any further information about how this works? I'd like to
customize the fields that are printed. I'd also like to include the
"From:" field, as only the sender (envelope-from) appears to be
available.

Any ideas would be *greatly* appreciated.
Thanks,
Alex