docs(tabs): adds README file (#527)

Author: robertmesserle

src/components/tab-group/README.md

@@ -0,0 +1,30 @@
+# MdTabGroup
+Tab groups allow the user to organize their content by labels such that only one tab is visible at any given time.
+
+### Examples
+A basic tab group would have the following markup.
+```html
+<md-tab-group>
+  <md-tab>
+    <template md-tab-label>One</template>
+    <template md-tab-content>
+      <h1>Some tab content</h1>
+      <p>...</p>
+    </template>
+  </md-tab>
+  <md-tab>
+    <template md-tab-label>Two</template>
+    <template md-tab-content>
+      <h1>Some more tab content</h1>
+      <p>...</p>
+    </template>
+  </md-tab>
+</md-tab-group>
+```
+
+## `<md-tab-group>`
+### Properties
+
+| Name | Type | Description |
+| --- | --- | --- |
+| `selectedIndex` | `number` | The index of the currently active tab. |