Add notes on installing other types.

Author: chriskrycho

README.md

@@ -10,6 +10,7 @@ Use TypeScript in your Ember 2.x and 3.x apps!
   * [`tsconfig.json`](#tsconfigjson)
 * [Using TypeScript with Ember effectively](#using-typescript-with-ember-effectively)
   * [Incremental adoption](#incremental-adoption)
+  * [Install other types!](#install-other-types)
   * [Environment configuration typings](#environment-configuration-typings)
   * [Service and controller injections](#service-and-controller-injections)
   * [Opt-in unsafety for Ember Data lookups](#opt-in-unsafety-for-ember-data-lookups)
@@ -102,6 +103,15 @@ You may find the blog series ["Typing Your Ember"][typing-your-ember] helpful as
 
 [typing-your-ember]: http://www.chriskrycho.com/typing-your-ember.html
 
+### Install other types!
+
+You'll want to use other type definitions as much as possible. The first thing you should do, for example, is install the types for your testing framework of choice: `@types/ember-mocha` or `@types/ember-qunit`. Beyond that, look for types from other addons: it will mean writing `any` a lot less and getting a lot more help both from your editor and from the compiler.
+
+To make this easier, we're maintaining [a list of addons with known type definitions][known-typings] either on [Definitely Typed] or as part of the addon itself. (If you know of typings that aren't in that list, please open a pull request to add them!)
+
+[known-typings]: ./known-typings.md
+[definitely typed]: https://github.com/DefinitelyTyped/DefinitelyTyped
+
 ### Environment configuration typings
 
 Along with the @types/ files mentioned above, ember-cli-typescript adds a starter interface for `config/environment.js` in `config/environment.d.ts`. This interface will likely require some changes to match your app.