docs: ubuntu iso's hybrid format and parted warnings (#585)

Document that `parted` warnings about invalid GPT and physical block
size issues can be safely ignored as they are a consequence of "hybrid
iso's".

Fixes: https://github.com/resin-io/etcher/issues/312
Signed-off-by: Juan Cruz Viotti <jviottidc@gmail.com>

docs/USER-DOCUMENTATION.md

@@ -16,4 +16,35 @@ To deactivate this feature, `touch` any of the files listed below:
 - `/usr/share/appimagekit/no_desktopintegration`
 - `/etc/appimagekit/no_desktopintegration`
 
+Flashing Ubuntu ISOs
+--------------------
+
+Ubuntu images (and potentially some other related GNU/Linux distributions) have
+a peculiar format that allows the image to boot without any further
+modification from both CDs and USB drives.
+
+A consequence of this enhancement is that some programs, like `parted` get
+confused about the drive's format and partition table, printing warnings such
+as:
+
+> /dev/xxx contains GPT signatures, indicating that it has a GPT table.
+> However, it does not have a valid fake msdos partition table, as it should.
+> Perhaps it was corrupted -- possibly by a program that doesn't understand GPT
+> partition tables.  Or perhaps you deleted the GPT table, and are now using an
+> msdos partition table.  Is this a GPT partition table?  Both the primary and
+> backup GPT tables are corrupt.  Try making a fresh table, and using Parted's
+> rescue feature to recover partitions.
+
+***
+
+> Warning: The driver descriptor says the physical block size is 2048 bytes,
+> but Linux says it is 512 bytes.
+
+All these warnings are **safe to ignore**, and your drive should be able to
+boot without any problems.
+
+Refer to [the following message from Ubuntu's mailing
+list](https://lists.ubuntu.com/archives/ubuntu-devel/2011-June/033495.html) if
+you want to learn more.
+
 [appimage]: http://appimage.org