docs(tabs): add overview (#2336)

Author: andrewseguin

src/lib/tabs/OVERVIEW.md

@@ -0,0 +1,60 @@
+The Angular Material tab group helps organize content into separate views where only one view can be
+visible at a time. Each tab's label is shown in the tab header and the active
+tab's label is designated with the animated ink bar. When the list of tab labels exceeds the width
+of the header, pagination controls appear to let the user scroll left and right across the labels.
+
+The active tab may be set using the `selectedIndex` input or when the user selects one of the
+tab labels in the header.
+
+<!-- example(tab-overview) -->
+
+### Events
+
+The `selectChange` output event is emitted when the active tab changes.  
+
+The `focusChange` output event is emitted when the user puts focus on any of the tab labels in
+the header, usually through keyboard navigation. 
+
+### Labels
+
+If a tab's label is only text then the simple tab-group API can be used.
+
+```html
+<md-tab-group>
+  <md-tab label="One">
+    <h1>Some tab content</h1>
+    <p>...</p>
+  </md-tab>
+  <md-tab label="Two">
+    <h1>Some more tab content</h1>
+    <p>...</p>
+  </md-tab>
+</md-tab-group>
+```
+
+For more complex labels, add a template with the `md-tab-label` directive inside the `md-tab`.
+
+```html
+<md-tab-group>
+  <md-tab>
+    <template md-tab-label>
+      The <em>best</em> pasta
+    </template>
+    <h1>Best pasta restaurants</h1>
+    <p>...</p>
+  </md-tab>
+  <md-tab>
+    <template md-tab-label>
+      <md-icon>thumb_down</md-icon> The worst sushi
+    </template>
+    <h1>Terrible sushi restaurants</h1>
+    <p>...</p>
+  </md-tab>
+</md-tab-group>
+```
+
+### Dynamic Height
+
+By default, the tab group will not change its height to the height of the currently active tab. To
+change this, set the `dynamicHeight` input to true. The tab body will animate its height according
+ to the height of the active tab.