Created Skim document (markdown)

Skim-document.md

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+[SkimToWidget](https://github.com/koreader/koreader/blob/master/frontend/ui/widget/skimtowidget.lua) is a multifunctional book navigation tool. It can be invoked with:
+- reader menu `Navigation` – `Skim document`;
+- `Skim` button in the `Go to page` dialog;
+- a long press on the bottom status bar, if enabled;
+- an assigned gesture.  
+![SkimToWidget](https://user-images.githubusercontent.com/62179190/111024709-e4404280-83e8-11eb-94d1-9d09659f4f74.png)
+---
+  
+**Top row buttons** actions:  
+`▕◁` **press**: go to the previous chapter; **hold**: go to the first page of the book  
+`◁☆` **press**: go to the previous bookmark; **hold**: go to the first bookmark  
+`☆` **press**: toggle bookmark; **hold**: show the list of bookmarks  
+`☆▷` **press**: go to the next bookmark; **hold**: go to the last bookmark  
+`▷▏` **press**: go to the next chapter; **hold**: go to the last page of the book  
+
+Tap the **progress bar** to go to the desired location in the book.  
+Reader menu `Navigation` - `Settings` – `Progress bars` sets the ToC level for ticks shown in the progress bar.
+ 
+**Bottom row buttons** actions on press:  
+`-10` go to 10 pages backward  
+`-1` go to one page backward  
+`##` invoke `Go to page` dialog  
+`+1` go to one page forward  
+`+10` go to 10 pages forward  
+ 
+SkimToWidget is a [movable element](https://github.com/koreader/koreader/pull/3636).  
+Hold the widget outside of the top row buttons to toggle its transparency.
+
+Tap the screen outside of the SkimToWidget to close it.