Follow-up from "hot reloading for .vue files"

The following discussion from gitlab-ce!12180 should be addressed:

• @mikegreiling started a discussion:

  It doesn't have to be done in this MR, but can you please open up a follow up issue to add gitlab-ce~2278655 for HMR in our frontend development guidelines?

  We ought to link to the official documentation for webpack and vue-loader respectively:
  https://webpack.js.org/guides/hot-module-replacement/
  https://vue-loader.vuejs.org/en/features/hot-reload.html

  If we end up adopting Vuex (gitlab-ce#33985) we ought to note some of the limitations there as well:
  https://vuex.vuejs.org/en/hot-reload.html

/cc @simon