chore: make it more obvious theming setup is required (#1447)

Author: jelbourn

GETTING_STARTED.md

@@ -33,27 +33,29 @@ import { MaterialModule } from '@angular/material';
 export class PizzaPartyAppModule { }
 ```
 
-### Including core and theme styles:
-See the [theming guide](docs/theming.md) for more information.
+## Including core and theme styles:
+This is required to apply all of the core and theme styles to your application.
+See the [theming guide](docs/theming.md) for instructions.
 
 
-### Additional setup for `md-slide-toggle` and `md-slider`:
+## Additional setup for `md-slide-toggle` and `md-slider`:
 The slide-toggle and slider components have a dependency on [HammerJS](http://hammerjs.github.io/).
 
 Add HammerJS to your application via [npm](https://www.npmjs.com/package/hammerjs), a CDN 
 (such as the [Google CDN](https://developers.google.com/speed/libraries/#hammerjs)), or served 
 directly from your app.
 
-### [Optional] Using Material Design icons with `md-icon`:
+## [Optional] Using Material Design icons with `md-icon`:
 
-- If you want to use Material Design icons, load the Material Design font in your `index.html`.  
-`md-icon` supports any font icons or svg icons, so this is only one potential option.
+- If you want to use Material Design icons in addition to Angular Material components, 
+load the Material Design font in your `index.html`.  
+`md-icon` supports any font icons or svg icons, so this is only one option for an icon source.
 
 **src/index.html**
 ```html
 <link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
 ```
 
-### Sample Angular Material 2 projects
+## Sample Angular Material 2 projects
 - [Material 2 Sample App](https://github.com/jelbourn/material2-app)
 - [Angular Connect 2016 Demo](https://github.com/kara/leashed-in)