Keymanagment: new Buffer() is deprecated, use new Buffer.from() instead

[danlutz]

The docs on "Key Managment" should use

const keypair = btcpay.crypto.load_keypair(new Buffer.from(<privateKey>, 'hex'))

instead of

var keypair = btcpay.crypto.load_keypair(new Buffer(privateKey, "hex"))

I wanted to create a pull request, but either I do not have the right to do so or I could not find out how (First time 😛 )

[danlutz]

Update: I just created a pull request for this