Update README.rdoc

Update readme with right rails `cache_store` config.
Also add `3.x` because the config doesnt work on rails4, as the mem_cache_store adapter now only works with dalli. see https://github.com/rails/rails/blob/4-0-stable/activesupport/lib/active_support/cache/mem_cache_store.rb and https://github.com/rails/rails/blob/4-0-stable/activesupport/lib/active_support/cache/mem_cache_store.rb#L141 which breaks the memcached gem API.

(I am working on https://github.com/Shopify/rails_cache_adapters to make an adapter to memcached + rails4), after I finish it, I can update the instructions to use it on rails 4.