docs(GUI): explain why some drives are not bootable out of the box

The usual case is images requiring special treatment. We recommend the
users to use specialised tooling for those cases.

Signed-off-by: Juan Cruz Viotti <jviotti@openmailbox.org>

docs/USER-DOCUMENTATION.md

@@ -3,6 +3,20 @@ Etcher User Documentation
 
 This document contains how-tos and FAQs oriented to Etcher users.
 
+Why is my drive not bootable?
+-----------------------------
+
+Etcher copies images to drives byte by byte, without doing any transformation
+to the final device, which means images that require special treatment to be
+made bootable, like Windows images, will not work out of the box. In these
+cases, the general advice is to use software specific to those kind of
+images, usually available from the image publishers themselves.
+
+Images known to require special treatment:
+
+- Microsoft Windows (use [Windows USB/DVD Download Tool][windows-usb-tool], or
+  [Rufus][rufus]).
+
 Deactivate desktop shortcut prompt on GNU/Linux
 -----------------------------------------------
 
@@ -192,3 +206,5 @@ platforms.
 [electron]: http://electron.atom.io
 [electron-supported-platforms]: https://github.com/electron/electron/blob/master/docs/tutorial/supported-platforms.md
 [etcher-cli]: https://github.com/resin-io/etcher/blob/master/docs/CLI.md
+[windows-usb-tool]: https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
+[rufus]: https://rufus.akeo.ie