Move scope wiki page as Core module doc

lib/datagrid/core.rb

@@ -3,8 +3,46 @@
 require "datagrid/drivers"
 require "active_support/core_ext/class/attribute"
 require "active_model/attribute_assignment"
-
 module Datagrid
+  # ## Simple example
+  #
+  # Datagrid scope as assets source to be queried from the database.
+  # In most cases it is a model class with some default ORM scopes like `order` or `includes`:
+  #
+  # ``` ruby
+  # class ProjectsGrid
+  #  include Datagrid
+  #  scope { Project.includes(:category) }
+  # end
+  # ```
+  #
+  # Scope is also used to choose a ORM driver(Mongoid, ActiveRecord, etc.), get wether filters and columns defined below has order.
+  #
+  # You can set scope at instance level:
+  #
+  # ``` ruby
+  # grid = ProjectsGrid.new(grid_params) do |scope|
+  #   scope.where(owner_id: current_user.id)
+  # end
+  #
+  # grid.assets # => SELECT * FROM projects WHERE projects.owner_id = ? AND [other filtering conditions]
+  # ```
+  #
+  # Scope can always be retrieved and redefined at instance level:
+  #
+  # ``` ruby
+  # grid.scope # => SELECT * FROM projects WHERE projects.user_id = ?
+  # grid.redefined_scope? # => true
+  #
+  # # Reset scope to default class value
+  # grid.reset_scope
+  # grid.assets # => SELECT * FROM projects
+  # grid.redefined_scope? # => false
+  #
+  # # Overwriting the scope (ignore previously defined)
+  # grid.scope { current_user.projects }
+  # grid.redefined_scope? # => true
+  # ```
   module Core
     include ::ActiveModel::AttributeAssignment