Update deployment-faq.md

[gone-to-the-future]

Add note about testnet version

[CLAassistant]

All committers have signed the CLA.

[MrNaif2018]

Hi, what do you mean by bitcoin testnet v3? I mean we support all kinds of testnets, they may be named differently. But testnet we enable is the one you see in all testnet blockchain explorers, so why would it matter?

[gone-to-the-future]

There are multiple Bitcoin testnets. v4 is the main one used by devs currently, but v3 is still supported (but deprecated). If you send/receive coins on the wrong testnet, they won't show up in Bitcart. If somebody is trying to develop w Bitcart using testnet, they need to know which testnet Bitcart uses.

This took me some time to figure out, I just want to help future devs.

Addl info: https://en.bitcoin.it/wiki/Testnet

[gone-to-the-future]

FWIW Electrum supports testnet 3 and testnet 4, depending on how it is launched. I assume Bitcart launches with --testnet which enables v3, but --testnet4 is also an option