gitbooks not showing link for "Core concepts" (#482)

* gitbooks not showing link for "Core concepts"

While reading the online book there is no link for next chapter after the Getting started one because Core concepts has no file attached to it.

* Create core-concepts.md

* added link to core-concepts.md

* Update README.md

* Update core-concepts.md

Author: thewhitetulip

docs/en/README.md

@@ -1 +1 @@
-{% include "./SUMMARY.md" %}
+{% include "./SUMMARY.md" %}

docs/en/SUMMARY.md

@@ -9,7 +9,7 @@
 - [Installation](installation.md)
 - [What is Vuex?](intro.md)
 - [Getting Started](getting-started.md)
-- Core Concepts
+- [Core Concepts](core-concepts.md)
   - [State](state.md)
   - [Getters](getters.md)
   - [Mutations](mutations.md)

docs/en/core-concepts.md

@@ -0,0 +1,12 @@
+# Core Concepts
+
+We will be learning the core concepts in Vuex in this chapters. They are
+  - [State](state.md)
+  - [Getters](getters.md)
+  - [Mutations](mutations.md)
+  - [Actions](actions.md)
+  - [Modules](modules.md)
+
+A deep understanding of all these concepts is essential for using vuex. 
+
+Let's get started.