docs(list): add docs for subheaders and dividers in lists

src/components/list/README.md

@@ -107,22 +107,22 @@ Output:
 
 ### Lists with multiple sections
 
-You can add a subheader to a list by wrapping your subheader text in `<md-subheader>` tags, and you can 
-add dividers with `<md-divider>` tags.
+You can add a subheader to a list by annotating a heading tag with an `md-subheader` attribute. To add a divider,
+use `<md-divider>` tags.
 
 ```html
 <md-list>
-   <md-subheader>Folders</md-subheader>
+   <h3 md-subheader>Folders</h3>
    <md-list-item *ngFor="#folder of folders">
       <i md-list-avatar class="material-icons">folder</i>
-      <h3 md-line>{{folder.name}}</h3>
+      <h4 md-line>{{folder.name}}</h4>
       <p md-line class="demo-secondary-text"> {{folder.updated}} </p>
    </md-list-item>
    <md-divider></md-divider>
-   <md-subheader>Notes</md-subheader>
+   <h3 md-subheader>Notes</h3>
    <md-list-item *ngFor="#note of notes">
       <i md-list-avatar class="material-icons">note</i>
-      <h3 md-line>{{note.name}}</h3>
+      <h4 md-line>{{note.name}}</h4>
       <p md-line class="demo-secondary-text"> {{note.updated}} </p>
    </md-list-item>   
 </md-list>