Merge pull request #16 from JasinYip/doc/build-api-return-promise

doc: mpvueSimple.build returns a Promise now!

Author: aOrz

docs/mpvue/simple.md

@@ -45,6 +45,11 @@ mpvueSimple.build({
   output: 'mp-pages',
   pageName: 'login'
 })
+
+// maybe you want to do something after building
+mpvueSimple.build()  // => Promise
+.then(() => console.log('mpvue build success'))
+.catch(err => throw new Error(err))
 ```
 
 #### API