Installation docs should mention encrypted android device workaround

If you've encrypted your android device, there's one more step you must perform before you can install Ubuntu on it. You must perform a factory reset via the android system settings. Otherwise, even if you use `ubuntu-device-flash ... --bootstrap` you'll still end up with android on your device (despite the fact that when your device reboots, you'll get the spinny Ubuntu logo). A factory reset is the only way to "undo" device encryption. This might be a useful URL to link to:

http://www.howtogeek.com/141953/how-to-encrypt-your-android-phone-and-why-you-might-want-to/

I suggest adding this as a small section under "Unlocking the Android device". Note that as with other advice here, the user may want to back up their android data before factory resetting.

Reported from: http://developer.ubuntu.com/start/ubuntu-for-devices/installing-ubuntu-for-devices/