docs: Added a mixin to TS example (apertureless#593)

README.md

@@ -111,11 +111,12 @@ or in TypeScript
 
 ```ts
 // CommitChart.ts
-import { Bar } from 'vue-chartjs';
+import { Bar, mixins } from 'vue-chartjs';
 import { Component } from 'vue-property-decorator';
 
 @Component({
     extends: Bar, // this is important to add the functionality to your component
+    mixins: [mixins.reactiveProp]
 })
 export default class CommitChart extends Vue<Bar> {
   mounted () {