Document how to customize base dir

[ggarnier]

Adds a description on how to change the base directory where phantomjs will be installed.

[tamaloa]

We use this to be able to cache phantomjs binaries during CI builds on Gitlab (caching is limited to dirs under project root). Note that including the phantomjs version is also really easy:

require 'phantomjs'
Phantomjs.base_dir = "#{Rails.root}/vendor/phantomjs/#{Phantomjs.version}"
require 'phantomjs/poltergeist'