Add tracked_seconds to task

Author: Tambet Masik

chapters/tasks.md

@@ -10,15 +10,10 @@ Task has the following properties
 * estimated_seconds: estimated duration of task in seconds (integer, not required)
 * active: whether the task is done or not (boolean, by default true)
 * at: timestamp that is sent in the response for PUT, indicates the time task was last updated
+* tracked_seconds: total time tracked (in seconds) for the task
 
 Workspace id (wid) and project id (pid) can't be changed on update.
 
-###Additional fields###
-It's possible to get additional info for the task. For that you have to send the `fields` parameter in request with desired property names separated by comma.
-
-* done_seconds: duration (in seconds) of all the time entries registered for this task
-* uname: full name of the person to whom the task is assigned to
-
 ##Actions for single project user##
 ###Create a task###
 
@@ -168,4 +163,4 @@ curl -v -u 1971800d4d82861d8f2c1651fea4d212:api_token \
 	-X DELETE https://www.toggl.com/api/v8/tasks/1335076912,1335076911,1335076910
 ```
 
-Successful request will return `200 OK`. If the user has no access to delete, you'll get a status code `4xx`
+Successful request will return `200 OK`. If the user has no access to delete, you'll get a status code `4xx`