Add <select> guidance

I recently used a `` tag in an Ember form and wanted to do so without an addon. I discovered that there's no good source of information on how to do so properly. Reading forum posts and Googling [_ember select_](https://www.google.com/search?q=ember+select) gives:

* Really old stuff on the `select` view
* The [ember-power-select](https://github.com/cibernox/ember-power-select) and [x-select](https://github.com/thefrontside/emberx-select) addons
* [A couple](https://balinterdi.com/blog/how-to-do-a-select-dropdown-in-ember-20/) [blog posts](https://medium.com/ember-ish/easy-select-element-in-ember-js-dde318c07110) from 2015 and 2016 that I believe suggest the correct non-addon approach.

Using the `` tag is super common, and it feels wrong to have to use addons or read blog posts to learn out how to use it.

I'd suggest adding a short explanation to the Input Helpers section; even though there isn't a helper for it, I think that's where most people would end up looking. I'd suggest using a shortened adaptation of [Jen's post](https://medium.com/ember-ish/easy-select-element-in-ember-js-dde318c07110) that is one of the top Google hits for _ember select_.

An alternative could be to create an RFC for making the `x-select` addon part of Ember, like the `{{input}}` and `{{textarea}}` helpers are. I believe including a built-in `` helper was rejected a long time ago when the `select` view was deprecated, but I don't know the details.

If this is acceptable, I'll make a PR.