Add a section on promises to the JS primer

[jenweber]

https://guides.emberjs.com/v3.0.0/getting-started/js-primer/

Promises are an important part of Ember, but they aren't a "beginner" JS concept. We need to add a section that explains how to use them in the context of Ember. You can't get too in the weeds - a dev needs to do their own research into JS basics - but we can't just assume that people will catch on quickly. The explanation should be similar in length to other sections (like let vs var) and include a code sample, such as a createRecord/save and then.

Contributors tackling this should expect some iteration after making a PR. It's a tough balance to strike.

Note, this issue is set aside for the EmberConf contributors workshop on 3/12. If it's not claimed by the end of that day, feel free to grab it!

[karan-pathak]

Hi @jenweber, here's the first draft https://github.com/emberjs/guides/pull/2317. I would love to know your thoughts on it :)