Document how to use regtest

[benma]

One can setup a regtest wallet, but docs for how to actually get coins into the wallet, generate blocks etc would be very helpul.

I assume bitcoin-cli is needed, but that does not seem to be automatically downloaded alongside bitcoind. That would be helpful as well.

[pythcoiner]

note: i wrote this tool to control a regtest:
https://github.com/pythcoiner/Minta

[pythcoiner]

related to #1159