docs(MdCheckbox): Create README for MdCheckbox.

Closes #185

src/components/checkbox/README.md

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+# md-checkbox
+
+`md-checkbox` is a Material Design selection control that allows users to make a binary choice for
+a predetermined conditioned. It is modeled after the browser's native checkbox element, and behaves
+in the same way. Similar to the native checkbox element, it supports an indeterminate state for
+"mixed" checkboxes.
+
+A demo of the checkbox can be found at https://plnkr.co/edit/P7qce8lN9n2flS6kBhDy?p=preview.
+
+## Usage
+
+### Basic Usage
+
+`md-checkbox` can be used anywhere a normal checkbox would be used, and in the same way.
+
+```html
+<ul>
+  <li *ngFor="#todo of todos">
+    <md-checkbox [checked]="todo.completed"
+                 (change)="todo.completed = $event">
+      {{todo.name}}
+    </md-checkbox>
+  </li>
+</ul>
+```
+
+### Usage within Forms
+
+In addition to supporting native checkbox functionality, `md-checkbox` also supports `[(ngModel)]`
+and `ngControl` for use within forms.
+
+```html
+<form (submit)="saveUser()">
+  <!-- Form fields... -->
+  <div>
+    <md-checkbox [(ngModel)]="user.agreesToTOS">
+      I have read and agree to the terms of service.
+    </md-checkbox>
+  </div>
+  <button type="submit" [disabled]="!user.agreesToTOS">Sign Up</button>
+</form>
+```
+
+### Indeterminate Checkboxes
+
+Indeterminate checkboxes are useful when a checkbox needs to be in a "mixed" state
+
+```html
+<md-checkbox [checked]="false"
+             [indeterminate]="isIndeterminate"
+             (change)="isIndeterminate = false">
+  Click the Button Below to Make Me Indeterminate.
+</md-checkbox>
+<button type="button" (click)="isIndeterminate = true">
+  Make Indeterminate
+</button>
+```
+
+### Alignment
+
+Note that checkboxes can be aligned to come at the "start" or the "end" of its corresponding label.
+
+```html
+<md-checkbox [checked]="true" align="end">
+  I come after my label.
+</md-checkbox>
+```
+
+Note that this alignment is preserved within RTL layouts.
+
+### Accessibility
+
+By default, `md-checkbox` provides all the accessibility attributes needed. It also supports
+keyboard navigation and toggling via the spacebar. However, you can provide an `aria-label` to the
+checkbox if you do not wish to have any label text.
+
+```html
+<md-checkbox [checked]="false" aria-label="My standalone checkbox"></md-checkbox>
+```