Improved README.md - Mention Domain for ISO Download

[hl2guide]

Please update README.md to mention that:

Users may need to whitelist the domain vlscppe.microsoft.com, depending on your network configuration.
If blocked the error shows as:

Error: The requested name is valid, but no data of the requested type was found. (vlscppe.microsoft.com:443)

[pbatard]

I think the error is pretty explicit, and since the script can change at any moment and new domain requests added/removed, I'm not planning to hold the hand of people whose networks have nonstandard configurations.