Document setting up Vagrant-docker with the remote API

Hi,

I was having trouble setting up the remote HTTP API on my machine and I previously opened an issue asking how to set this up. In this PR I document what @pyotr777 explained in #2280.

I absolutely had no idea where to document this other than to put it in **contrib/vagrant** as recommended by @shykes, so if I did this wrong I'll be happy to amend this PR.