Add count_votes_* methods to readme cache section

I've been thankfully using this gem for years, but only now noticed there are `count_votes_*` methods that correctly chooses between the cached or non-cached method. Would be nice to see this reflected in the readme.

Ran into this issue when i noticed that `votes_for.size` never uses the cache. See also #82. Then i took a look in the source and noticed the `count_votes*` methods.
