Add N plus one operation solution section to README

Introduced a new section in the README to document for introduction of Grape::Entity::Preloader

Author: OuYangJinTing

README.md

@@ -34,6 +34,7 @@
     - [Entity Organization](#entity-organization)
     - [Caveats](#caveats)
   - [Installation](#installation)
+  - [N plus one operation solution](#n-plus-one-operation-solution)
   - [Testing with Entities](#testing-with-entities)
   - [Project Resources](#project-resources)
   - [Contributing](#contributing)
@@ -688,6 +689,10 @@ Or install it yourself as:
 
     $ gem install grape-entity
 
+## N plus one operation solution
+
+[Grape::Entity::Preloader](https://github.com/OuYangJinTing/grape-entity-preloader) allows preload associations and callbacks for avoiding N+1 operations in Grape::Entity.
+
 ## Testing with Entities
 
 Test API request/response as usual.