Add playground in example

Author: jeemyeong

example/build.gradle

@@ -40,6 +40,7 @@ dependencies{
     compile(project(":graphql-spring-boot-starter"))
     compile(project(":graphiql-spring-boot-starter"))
     compile(project(":voyager-spring-boot-starter"))
+    compile(project(":playground-spring-boot-starter"))
 
     compile("org.springframework.boot:spring-boot-starter-web:$LIB_SPRING_BOOT_VER")
     compile("org.springframework.boot:spring-boot-starter-actuator:$LIB_SPRING_BOOT_VER")

example/src/main/resources/application.yml

@@ -26,4 +26,8 @@ graphiql:
 voyager:
   enabled: true
   cdn:
-    enabled: false
+    enabled: false
+graphql.playground:
+  enabled: true
+  cdn:
+    enabled: false