Update documentation with instructions for Rails.cache

[xtr3me]

After upgrading we noticed that the configuration we had set originally resulted in a crash (which worked on version 1.6.3 but not after updating to 1.7.2):

Geocoder.configure(cache: Rails.cache)
ArgumentError: wrong number of arguments (given 1, expected 0)
from <abbreviated>/gems/activesupport-5.2.6/lib/active_support/cache/redis_cache_store.rb:166:in `initialize'

Initially I wrote a simple adapter around Rails.cache but when I was preparing a PR I noticed that we could use the Generic cache store wrapper to work with Rails.cache.

This PR helps the developer who uses Rails.cache to setup Geocoder's caching functionality.
It also instructs the developer to use the non deprecated way of setting the cache prefix as it was moved.

This was the adapter I initially wrote:

module Geocoder::CacheStore
  class RailsCache < Base
    def initialize(store = nil, options = {})
      super
      @store = ::Rails.cache
      @expiration = options[:expiration]
    end

    def write(url, value, expire = @expiration)
      if expire.present?
        store.write(key_for(url), value, expires_in: expire)
      else
        store.write(key_for(url), value)
      end
    end

    def read(url)
      store.read key_for(url)
    end

    def remove(key)
      store.delete(key)
    end

    private

    def expire
      @expiration
    end
  end
end

[alexreisner]

This is excellent. Thanks very much!