Update required column config for children_count

In order for this feature to function properly when we add counter_cache
updating on updating (currently it's only for create/delete) we'll need
these values to be consistent in the database.

Without a default of 0 if we tried to `increment_counter` on `nil` we
would run into errors.

README.md

@@ -26,6 +26,8 @@ gem 'awesome_nested_set'
 To make use of `awesome_nested_set` your model needs to have 3 fields:
 `lft`, `rgt`, and `parent_id`. The names of these fields are configurable.
 You can also have optional fields: `depth` and `children_count`. These fields are configurable.
+Note that the `childrent_count` column must have `null: false` and `default: 0` to
+function properly.
 
 ```ruby
 class CreateCategories < ActiveRecord::Migration
@@ -66,7 +68,7 @@ You can pass various options to `acts_as_nested_set` macro. Configuration option
 * `left_column`: column name for left boundary data (default: lft)
 * `right_column`: column name for right boundary data (default: rgt)
 * `depth_column`: column name for the depth data default (default: depth)
-* `scope`: restricts what is to be considered a list. Given a symbol, it'll attach “_id” (if it hasn't been already) and use that as the foreign key restriction. You can also pass an array to scope by multiple attributes. Example: `acts_as_nested_set :scope => [:notable_id, :notable_type]`
+* `scope`: restricts what is to be considered a list. Given a symbol, it'll attach `_id` (if it hasn't been already) and use that as the foreign key restriction. You can also pass an array to scope by multiple attributes. Example: `acts_as_nested_set :scope => [:notable_id, :notable_type]`
 * `dependent`: behavior for cascading destroy. If set to :destroy, all the child objects are destroyed alongside this object by calling their destroy method. If set to :delete_all (default), all the child objects are deleted without calling their destroy method. If set to :nullify, all child objects will become orphaned and become roots themselves.
 * `counter_cache`: adds a counter cache for the number of children. defaults to false. Example: `acts_as_nested_set :counter_cache => :children_count`
 * `order_column`: on which column to do sorting, by default it is the left_column_name. Example: `acts_as_nested_set :order_column => :position`

spec/db/schema.rb

@@ -46,7 +46,7 @@
  t.column :lft, :integer
  t.column :rgt, :integer
  t.column :depth, :integer
- t.column :children_count, :integer
+ t.column :children_count, :integer, null: false, default: 0
  end
 
  create_table :brokens, :force => true do |t|