add additional explanation of rollup issue

Author: mmalerba

src/material-moment-adapter/adapter/moment-date-adapter.ts

@@ -10,6 +10,9 @@ import {Inject, Injectable, Optional} from '@angular/core';
 import {DateAdapter, MAT_DATE_LOCALE} from '@angular/material';
 
 // Depending on whether rollup is used, moment needs to be imported differently.
+// Since Moment.js doesn't have a default export, we normally need to import using the `* as`
+// syntax. However, rollup creates a synthetic default module and we thus need to import it using
+// the `default as` syntax.
 // TODO(mmalerba): See if we can clean this up at some point.
 import {default as _rollupMoment, Moment} from 'moment';
 import * as _moment from 'moment';