add hourly_rate and fixed_rate to timesheet entries (#302)

var/docs/timesheet.md

@@ -17,3 +17,17 @@ The `duration` field supports entering data in the following formats:
 | Seconds | {seconds} | | `3600` = 1 Hour / `8820` = 2 Hours, 27 Minutes |
 
 Please note: if time rounding is activated (which is the default behaviour), then your entered seconds might be removed after submitting the form.
+
+## Rate calculation
+
+The rate of a timesheet entry can be calculated from several settings.
+
+It can have two values dedicated to the entry itself:
+- if a fixed rate is set, the rate of a record is set to this exact value no matter how long the duration is
+- if a hourly rate is set, it will be used to calculate the record rate by using it multiplied with the records duration
+- each of the above can be set to 0, to set the records rate to 0
+
+If none of the above was set the users rate will be used to calculate the records rate.
+
+Please see also the configuration chapter about [hourly rates for timesheet records](configurations.md) to see how you 
+can apply configurable multiplying factors based on day and time.