Add instructions on resuming partial downloads

Resolves: QubesOS/qubes-issues#8563

Adding simple instructions on resuming downloads with for users
with unsteady Internet connections who are unfamiliar with wget.

user/downloading-installing-upgrading/installation-guide.md

@@ -43,7 +43,7 @@ Even on supported hardware, you must ensure that [IOMMU-based virtualization](ht
 
 ### Copying the ISO onto the installation medium
 
-Pick the most secure existing computer and OS you have available for downloading and copying the Qubes ISO onto the installation medium. [Download](/downloads/) a Qubes ISO.
+Pick the most secure existing computer and OS you have available for downloading and copying the Qubes ISO onto the installation medium. [Download](/downloads/) a Qubes ISO. If your Internet connection is unstable and the download is interrupted, you could resume the partial download with `wget --continue` in case you are currently using wget for downloading or use a download-manager with resume capability.
 
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