Add documentation about Review feature into User Guide (#2638)

cvat/apps/documentation/user_guide.md

@@ -35,6 +35,7 @@
   - [Annotation with cuboids](#annotation-with-cuboids)
   - [Annotation with tags](#annotation-with-tags)
   - [Track mode with polygons](#track-mode-with-polygons)
+  - [Review](#review)
   - [Automatic annotation](#automatic-annotation)
   - [Shape grouping](#shape-grouping)
   - [Filter](#filter)
@@ -290,6 +291,7 @@ Go to the [Django administration panel](http://localhost:8080/admin). There you
       - Duration — is the amount of time the job is being worked.
       - Assignee is the user who is working on the job.
         You can start typing an assignee’s name and/or choose the right person out of the dropdown list.
+      - Reviewer – a user assigned to carry out the review, read more in the [review](#review) section.
       - `Copy`. By clicking Copy you will copy the job list to the clipboard.
         The job list contains direct links to jobs.
 
@@ -753,6 +755,9 @@ Button assignment:
   - [MS COCO](http://cocodataset.org/#format-data)
   - [YOLO](https://pjreddie.com/darknet/yolo/)
 - `Open the task` — opens a page with details about the task.
+- `Request a review` - calls up the form to submit the job for a review, read more in the [review](#review)  section.
+- `Finish the job` - changes the status of the job to `completed` and returns to the task page without review.
+- `Submit the review` - (available during the review) calls up the form to submit a review, read more in the [review](#review) section.
 
 #### Save Work
 
@@ -813,11 +818,10 @@ Open the job info.
 
 ![](static/documentation/images/image144_detrac.jpg)
 
-- Job status: `annotation`, `validation` or `completed` task
-
 _Overview_:
 
 - `Assinger` - the one to whom the job is assigned.
+- `Reviewer` – a user assigned to carry out the review, read more in the [review](#review) section.
 - `Start Frame` - the number of the first frame in this job.
 - `End Frame` - the number of the last frame in this job.
 - `Frames` - the total number of all frames in the job.
@@ -860,6 +864,7 @@ Switching between user interface modes.
 |![](static/documentation/images/image170.jpg)|`Points` |[Annotation with points](#annotation-with-points) |
 |![](static/documentation/images/image176.jpg)|`Cuboid` |[Annotation with cuboids](#annotation-with-cuboids) |
 |![](static/documentation/images/image171.jpg)|`Tag` |[Annotation with tags](#annotation-with-tag) |
+|![](static/documentation/images/image195.jpg)|`Open an issue` |[Review](#review) (available only in review mode) |
 
 **Edit block** - contains tools for editing tracks and shapes.
 |Icon |Description |Links to section |
@@ -1416,6 +1421,38 @@ Polygons in the track mode allow you to mark moving objects more accurately othe
 There is no need to redraw the polygon every time using `Shift+N`,
 instead you can simply move the points or edit a part of the polygon by pressing `Shift+Click`.
 
+## Review
+
+A special mode to check the annotation allows you to point to an object or area in the frame containing an error.
+To go into review mode, you need to select `Request a review` in the menu and assign the user to run a check.
+
+![](static/documentation/images/image194.jpg)
+
+After that, the job status will be changed to `validation`
+and the reviewer will be able to open the task in review mode.
+Review mode is a UI mode, there is a special "issue" tool which you can use to identify objects
+or areas in the frame and describe the problem.
+
+- To do this, first click `open an issue` icon on the controls sidebar:
+
+  ![](static/documentation/images/image195.jpg)
+
+- Then click on an object in the frame to highlight the object or highlight the area by holding the left mouse button
+and describe the problem. The object or area will be shaded in red.
+- The created issue will appear in the workspace and in the `issues` tab on the objects sidebar.
+- After you save the annotation, other users will be able to see the problem, comment on each issue
+and change the status of the problem to `resolved`.
+- You can use the arrows on the issues tab to navigate the frames that contain problems.
+
+  ![](static/documentation/images/image196_detrac.jpg)
+
+- Once all the problems are marked, save the annotation, open the menu and select "submit the review". After that you'll see a form containing the verification statistics, here you can give an assessment of the job and choose further actions:
+    - Accept - changes the status of the job to `completed`.
+    - Review next – passes the job to another user for re-review.
+    - Reject - changes the status of the job to `annotation`.
+
+  ![](static/documentation/images/image197.jpg)
+
 ## Automatic annotation
 
 Automatic Annotation is used for creating preliminary annotations.