Update README.md for usage with friendly_id
Update README.md for #impressionist method

A quick fix for https://github.com/charlotte-ruby/impressionist/issues/138
Issue for mongoid

Queries is always on ObjectId type of imperssion_id, while model not restricts the type.
Thats can cause to saves in String format, hence not imperssions will be counted on counter_cache
Merge pull request #150 from kinopyo/patch-1

Update README.md for usage with friendly_id
Merge pull request #151 from kinopyo/patch-2

Update README.md for #impressionist method
Merge pull request #154 from zonkeynir/master

Issue for mongoid