fnordmetric-ui: added some documentation

Counters

--------

  data-fnordmetric

    must be "counter"

  data-gauge

    name of the gauge to be displayed

  data-at

    display this value at a certain time, may be a unix timestamp,

    a time definition like "-3hours" (3 hours ago), "-45m" (45m ago)

    or "-3600" (3600 seconds = 1 hour ago). You can also get a sum

    by setting this to e.g. "sum(-3hours)" which will give you the

    sum of the last thee hours. The Default is "now".

  data-autoupdate

    refresh this value every N seconds

  data-unit

    display the value with a unit (e.g. € or ms)

      "klass": "generic",

      "cmd": "values_for",

      "gauge": "order_sum",

      "offsets": ["s3600"],

      "at": "-5h",

      "widget_key": 123123123

    }