Merge pull request #19 from ssxlulu/master

Add overview of the component

README.md

 # ShardingScaling - ShardingSphere Scaling Out Component
 
+## Overview
+The following figure may clearly express this component's role:
+
+![scale out](https://user-images.githubusercontent.com/14773179/67069547-08760080-f1b0-11e9-8020-7092f37a4ff2.png)
+
+Supplementary instruction about the figure:
+
+1. Only migrate whole database, can't support to migrate specified tables only. And the target schema name must be same with the source schema;
+2. The process of migration splits into two step, history data migration and realtime data migration.
+
+   (1) During history data migration, we use 'select *' syntax to acquire the data, and use 'insert' syntax to migrate the data to the target schema;
+   
+   (2) During realtime data migration, we use binlog to migrate the data, and we mark the binlog position before migration.
+3. If the table in the source schema has primary key, we can migrate it concurrently using 'where condition'.
+                                                                            
 ## Requirement
 
 MySQL: 5.1.15 ~ 5.7.x