📝 Document the progress bar

docs/tutorial/progress-bar.md

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+# Progress Bar in Taskbar (Windows, macOS, Unity)
+
+On Windows a taskbar button can be used to display a progress bar. This enables
+a window to provide progress information to the user without the user having to
+switch to the window itself.
+
+On macOS the progress bar will be displayed as a part of the dock icon.
+
+The Unity DE also has a similar feature that allows you to specify the progress
+bar in the launcher.
+
+__Progress bar in taskbar button:__
+
+![Taskbar Progress Bar][taskbar-progress-image]
+
+All three cases are covered by the same API - the `setProgressBar()` method
+available on instances of `BrowserWindows`. Call it with a number between `0`
+and `1` to indicate your progress. If you have a long-running task that's
+currently at 63% towards completion, you'd call it with `setProgressBar(0.63)`.
+
+Generally speaking, setting the parameter to a value below zero (like `-1`)
+will remove the progress bar while setting it to a value higher than one
+(like `2`) will switch the progress bar to intermediate mode.
+
+See the [API documentation for more options and modes][setprogressbar].
+
+```javascript
+const { BrowserWindow } = require('electron')
+const myWindow = new BrowserWindow()
+
+win.setProgressBar(0.5)
+```
+
+[taskbar-progress-image]: https://cloud.githubusercontent.com/assets/639601/5081682/16691fda-6f0e-11e4-9676-49b6418f1264.png
+[setprogressbar]: ../api/browser-window.md#winsetprogressbarprogress