docs(button): more detail on MdAnchor. (#466)

Author: jelbourn

src/components/button/README.md

@@ -1,6 +1,13 @@
 # md-button
 
-`md-button` is an HTML `<button>` or `<a>` tag enhanced with styling and animation to match the [spec of a Material Design button](https://www.google.com/design/spec/components/buttons.html).
+`md-button` is an HTML `<button>` or `<a>` tag enhanced with styling and animation to match the
+[Material Design button spec](https://www.google.com/design/spec/components/buttons.html).
+
+Users should employ a button element (`<button>`) when clicking triggers some action in the current
+view *without navigating*. An anchor element (`<a>`) should be used when clicking *navigates*
+the user to another URL. Depending on which element is used, the component will either be an
+instance of `MdButton` or `MdAnchor`. Visually, the two are identical.
+
 
 ### Button types
 
@@ -103,4 +110,4 @@ Properties:
 | Name | Type | Description |
 | --- | --- | --- |
 | `color` | `"primary"|"accent"|"warn"` | The color palette of the button
-| `disabled` | boolean | Whether or not the button is disabled
+| `disabled` | boolean | Whether or not the button is disabled

src/components/button/button.ts

@@ -6,6 +6,7 @@ import {
   ChangeDetectionStrategy,
   ElementRef,
   Renderer,
+  Type,
 } from '@angular/core';
 
 // TODO(jelbourn): Ink ripples.
@@ -132,3 +133,6 @@ export class MdAnchor extends MdButton {
     }
   }
 }
+
+
+export const MD_BUTTON_DIRECTIVES: Type[] = [MdButton, MdAnchor];