Add 'timer' api docs (#339)

docs/reference/control/timer/seconds.md

@@ -0,0 +1,25 @@
+# seconds
+
+Get the amount of time counted by the timer in seconds.
+
+The timer count begins from `0` when you program starts or is [reset](/reference/control/timer/reset). The number of seconds returned also includes milliseconds if there is a fractional part of a second too.
+
+## Returns
+
+* a [number](/types/number) that is the amount of time elapsed, in seconds, since the timer was started or reset.
+
+## Example
+
+Find out how many seconds go by between presses of the `down` button on the brick.
+
+```blocks
+brick.buttonDown.onEvent(ButtonEvent.Pressed, function () {
+    brick.showValue("DownButtonTime", control.timer1.seconds(), 1)
+    control.timer1.reset()
+})
+```
+
+## See also
+
+[millis](/reference/control/timer/millis), [reset](/reference/control/timer/reset)
+