added about scopes

* Scope should be named as a subset of its superset of the DB records because it does "scoping".
* Scope names should be read as a sentence of natural language like `[pluralized model name] which are [scope name]`.
* Scope names which have arguments should also follow the above convention.
* Scope names should not contain its model name (scopes in User model should not contain "user" word in its name).