add overview for scope design doc

doc/design/scope.md

@@ -2,11 +2,12 @@
 
 ## Overview
 
-预期使用场景。
+Scope is an important concept in programming languages, which defines a program region that a set of bindings between names and entities applies. In a specific scope, a valid name is uniquely associated with an entity, such as a variable. And in another scope, this name may refer to other entity or nothing at all. It clearly restricts the visibility and validity of names in a program. Hence **Scope** is introduced to PaddlePaddle to manage variables in context. But different from the original abstract concept, Scope now becomes an object with two important attributes:
 
-引出Scope的两个属性。
-    1. Scope是Variable的Container
-    2. Scope可以共享
+- Scope is a container of variables
+- Scope can be inherited or shared
+
+A detailed explanation of these two attributes goes as following.
 
 ## Scope is a Container of Variables.