added comments about need values for time_ago in Vue

app/assets/javascripts/vue_pipelines_index/time_ago.js.es6

@@ -22,6 +22,7 @@
     },
     computed: {
       finishdate() {
+        // need started_at or created_at here
         const date = new Date(
           new Date(
             this.pipeline.details.finished_at
@@ -34,6 +35,7 @@
         );
       },
       runningdate() {
+        // need started_at or created_at at here
         const date = new Date(
           new Date().getTime() - new Date(this.pipeline.started_at).getTime()
         );